Brake Pedal goes to floor

-
DaveS

DaveS

Well-Known Member
Joined
Nov 17, 2016
Messages
52
Reaction score
2
Location
The Netherlands
Hello guys,

in my ''74 Plymouth Duster my brake pedal goes to the floor.

some extra info:

- its power brakes
- already bled all the brakes, new bleeders installed.
- new fluid
- pedal goes to the floor with engine on or off, doesnt matter
- master cilinder is 42 years old...

when pumping the pedal with engine off, the pedal gets firmer after 30+- pumps, still goes to floor. then when engine gets started it gets soft straight away.

when driving, and ramming the pedal all the way down, the rear wheels lock up..

i just dont seem to get any pressure under the pedal..

when having the lid off the master cilinder and pumping the brakes, there are 2 ''geisers'' coming up. isnt this pressure that is now coming up supposed to go into the calipers/drums?
 
If you are positive that there are no leaks (be sure to check all lines, wheel cylinders etc) then the most likely cause will be your master cylinder bypassing internally.
 
A rebuilt master cylinder is cheap or you can rebuild yours. I would start there.
 
If you pump it up but it goes back soft, it might be suffering from ED? Just kidding, if you are sure there is no air in the system,I would suspect the 42 year old master cylinder. And yes it is normal for the geysers with the cap off.
 
i was already planning on replacing the master cilinder, since it was already 42 years old.

thats never bad to replace
 
when i look on rockauto it has master cilinders for manual and for power brakes, i cant really tell the diffrence between them from the outside.

what are the differences between a manual and a power brake master cilinder?
 
Does it have a big vacuum canister that the MC bolts to? If yes it's power if it bolts to the firewall it's non power.
 
Put a master cylinder on it. Done.
 
Think about the 30 pumps and those geysers. They are telling you a story,along with the low-pedal hitting the floor.
During all that pumping, you are moving fluid down the lines. It is obviously going somewhere cuz the geysers are proof that it s returning.
If the m/c was internally bypassing, there would be little to no twin geysers.
Also, as Tb says; the geysers should be very small, typically just roiling the surface.
-A geyser that breaks free from the fluid surface on a DB car, ,from the chamber that operates the front brakes, is telling you there is compressed air in the system somewhere.
-A geyser from the chamber that operates the rear brakes is telling you that one of two things is going on back there, Either; 1) there is air in that system, or 2) the rear slack-adjusters need adjusting.
-The many,many pumps is also telling a story. Usually this is also pointing to the rear slack adjusters needing adjustment. All that pumping is moving the rear shoes slowly out to the drums. If the pedal returns the m/c piston faster than the fluid is able to return,then eventually the shoes get to the drums. If the front system was OK, then at this point you would have a hard pedal. But the second you release the pedal, whoosh, the fluid is returned by the shoe return springs, and then you get to start over.
- The low pedal,going to the floor, is an indication that the front system has completely failed.

So, in my estimation, you have two problems; 1) the rear slack adjusters need tightening up to eliminate the many,many pumps, and then 2) get the air out of the front system.
There is a wild card. If the flex hoses have gone soft, they will introduce there own bs into the diagnostics, by acting like expansion balloons. I have only rarely seen this, so I seldom check for it until I have exhausted all other possibilities.
But I would adjust the rear slack-adjusters first. The operation of the m/c kindof dictates this. The front-brake system is directly attached to the foot-pedal,whereas the rears are not.At least not usually. The rear brake-system is hydraulically coupled, by a small pocket of fluid in the m/c. When you apply pedal, the pedal drives the rearmost piston forward, which immediately shuts off the compensating port. Then the fluid is pushed down the line to the front brake calipers. As this is going on the pocket of fluid trapped between the front and rear systems, hydraulically pushes the frontmost piston forward, which then shuts off the front compensating port. The fluid in that chamber then moves out to the rear wheel cylinders. If the rear slack adjusters are just a little loose, then no brake action occurs back there until you push seriously hard and push out all the fluid from the frontmost chamber, and then maybe you get a lil action back there. But if the slack adjusters are really loose, then there may just not be enough fluid moving out, and then you get zero brakes back there. In your case,it seems like with enough pumping, the rears do finally get enough fluid. But the reason you can actually do this is cuz the front system is not energizing, and you may actually be activating the rear brake system by it's safety feature.
The m/c is designed such that in the event of a hydraulic failure in the front system, then the rearmost piston will travel forward, and a small extension on the front side of it will drive the frontmost piston forward to engage the rear brakes. The proof of this going on is the very low pedal height, and it's hitting the floor is proof that the rear slack adjusters need attention.
--So that's what twin geysers, a low pedal hitting the floor and multiple pumping, tells me.
-To recap; loose rear slack adjusters and air in the front system. With a minor possibility of air in the rear system.Adjust the rears first. Crank them hard, and back off 3 strokes(not notches), fix the fronts, and then reset the rears last.That's how I would attack this.

- except for one thing; I would ensure that the m/c itself is purged of air.And that is why we bench-bleed m/cs. Once the master is mounted it invariably does not sit level, an so air gets trapped inside its chambers this air has got to come out through the compensating ports or be driven all the way down the lines to the end points. But as you can clearly see. the m/c sits well above the level of those end points, and driving air downwards is hard enough but trying to keep it moving downwards is almost impossible. So by far, starting with a fully operational m/c is very important,as is keeping it fully functional during the bleed-procedure,which means; do not let air get into the chambers from the top!
OK so go get 'em and good luck!
 
Last edited:
when the pedal hits the floor, the brake light on the instrument panel lights up, does this mean something?
 
Hyup
It means I'm probably right.
It means the combination switch has sensed a complete failure in one of the two hydraulic systems and it is telling you about it.
Since you told us the rear brakes can lock up, obviously that end is working, so by process of elimination, the front system has failed.
Since you didn't mention brake fluid all over the floor, the failure is not due to a loss of fluid.
Since you have a geyser in both reservoirs, the M/C is working.
Since the pedal is soft and the geyser is tall, I see air trapped in the front system, and probably lots of it.
Since it takes lots of pumps to get a pedal hard enough to activate the rear system, I see poorly adjusted slack-adjusters back there.And the geyser proves it.

BTW as to brake line hook up at the M/C; On a factory system,the rearmost chamber, the one closest to the firewall, should be hooked up to the front brake system;operating the calipers at the front wheels.

Also, you asked about m/c differences. There are two main differences between m/cs for front disc cars and all-drum cars.
A)First is the reservoir sizes; the calipers have much larger chambers and so over the course of time as fluid moves out to keep the wearing pads against the rotors, more fluid is moving in a disc system than in a drum system. To prevent the front system from drawing air during the life of the pads, the reservoir associated with it needs to be larger than the reservoir associated with the drum end.
B)second is the inclusion of a small valve in the drum brake port. The little valve is called a residual-pressure valve, and it was originally installed to maintain a minimum 8 to 10 psi of hydraulic pressure in a drum wheel cylinder. This was supposed to keep the w/c from leaking by keeping the rubber cups in contact with the cylinder bores.
 
Last edited:
DaveS said:
when the pedal hits the floor, the brake light on the instrument panel lights up, does this mean something?
Click to expand...

Yep, it's a clue that there is air still in the system, or the master is only pumping fluid to the front or rear brakes only.
The light is activated by the brake fluid moving a small valve, and if the pressure isn't the same front and rear it moves the little valve off center and makes the warning light come on.

It actually could be the master cylinder, it just that most people can't get the air out of their brakes is why I said probably still air in them.

I see AJ answered right before I did. :D
Are your bleeders on the front brakes at the upper end of the calipers.
(some switch them around for torsion bars and that makes it where you have to take the calipers off to bleed them)
 
oh and the front brakes got changed maybe a month back, it was done by pushing the pistons in, the brakes always functioned good.. i did notice the pedal moving lower and lower before reaching the hard spot on the pedal as time progressed.
 
bleeders are on top of the calipers on the front, changed them last night.. thats why i wrote this post because i was so confused because i figured myself it would be air in the front brakes because the rear do lock up, oh and they lock up after 1 pump i tell you.

the fluid looked black and old, so when putting bleeders on the back and front, i bled them until the fresh fluid from the reservoir came out, so im pretty dang sure that there is no more air in the front..
 
DaveS said:
oh and the front brakes got changed maybe a month back, it was done by pushing the pistons in, the brakes always functioned good.. i did notice the pedal moving lower and lower before reaching the hard spot on the pedal as time progressed.
Click to expand...

Ah, then it could very well be the master.
Sometimes just trying to bleed them and having junk in the fluid can ruin the cup seal inside it and cause this.
It is always recommended when bleeding to try and not push the pedal more than it normally travels just because of that, but it's hard not to.

Also, the hard spot you spoke of is one end not working right and what you feel is the master piston coming up against the fluid for the working end.
This is also what is activating the light.

If you are pretty sure on the bleeding, I guess you might as well get a new master.
They are pretty cheap.
 
Well,I must have misunderstood; I thought the car needed multiple stabs to get the rear to work. It seems that was an error. Ok well that eliminates sloppy-loose slack adjusters then. Since the rears actually are doing all the work, obviously the fronts are not contributing. Since the rears are so touchy, and the pedal is low, this would seem to indicate that the rearmost piston has traveled all the way to the front, and is operating the rear brakes mechanically. When this happens, the compensating port for the front brakes, which is in the rearmost chamber gets blocked, and no more fluid can enter that line.
But additionally, you said the pedal hits the floor! These two situations can only occur simultaneously if the rear slack adjusters are in fact loose.So I would still start there. If you have a booster, there is supposed to be a big return spring in there which is supposed to push the pedal back up to its normally parked position.Without a booster,this job falls to the rear shoe-return springs. When they pull the shoes back, they force the fluid back up the line to the m/c and then thru the inter-chamber fluid, parks the pedal. Now with the front system failed, the inter-chamber fluid is not there so the pedal has no way of parking itself.
So now,I'm gonna argue that the non-parking pedal may be preventing the front brake system from working, due to the closed C-port.

So now if it was my car, I would clamp the front brake hoses, and if the pedal gets hard within a few strokes,by lifting the pedal back up to its parking spot with a toe under the pedal,then the air is upstream after the clamps. But if the pedal still won't park, I would redo the bench bleed, and bleed the front system somewhere else between the m/c and the clamps. If the pedal now gets hard, I would remove the clamps and proceed to the calipers. But if with the clamps on, AND the re-bench-bleed AND the re-bleed say at the flex-hoses, and the pedal still not hard, grudgingly I would replace the m/c.That's what I would do on my car with rubber flex-lines; But I would not tell you to clamp your lines on account of they say this can damage the lines internally and you might have a catastrophic failure at some future date. And on examination of how these lines are built, I can certainly see that. That's what people say.
But I tell you what, if you don't figure out some way to do this, you will probably have to replace the m/c. And if the new m/c makes no difference, then you are out the cash and no closer to solving the issue.
And I'll tell you one more thing; if you disconnect both steel lines from the m/c, bench-bleed it on the car, and then install plugs into the ports; A hard pedal now means the m/c is just fine. Conversely, a soft pedal now or a pedal that falls over time, guarantees the m/c is junk.
 
If the seal fails between the front and rear chambers of the MC, that will give you the symptoms that you described. Moving the MC piston as far as you did when bleeding the system may have caused that seal to fail. However, that does not explain why the rears lock up. As others said, it sounds like there is air still in the front system.

Make sure your bleeding technique is right. Have someone sit in the car, open a bleeder and immediately have them push the brake pedal SLOWLY to the floor and then HOLD it there while you close the bleeder. Then have them SLOWLY allow the pedal to come back up to the top.

Your technique of pushing the pistons back in the front calipers is correct. What brand of front brake pads did you use?

When you put on the new MC, make sure you follow the 'bench bleeding' procedure.

The difference between the manual and power brake MC's is the piston diameter.

And it is MANDATORY with a car this old to replace all 3 rubber lines. It is about 90% probable that one or all of them are now starting to have internal failures that you cannot see.
 
More Information for KEMPARTS D84

kemparts d84

my brakes always functioned good, and the bleeders were never openend to replace the brakes..

my bleeding skills are on point, tube on the bleeder, see the bubbles. in a bottle with a little fluid in it.. so if it sucks it up its sucks fresh fluid. snug fitting tube on the bleeder so no air can get in via that way.. the bleeders werent openend between the time the brakes functioned and when they stopped functioning.. after they stopped functioning i figured to change bleeders because somehow air got in.. still didnt fix it

im going to check for leaks again soon, and order a new master cilinder. they are not that expensive (shipping is even more expensive than the part itself) the current one is 42 year old so thats never a bad investment.

i will update this post of the results of the new master cilinder.
 
Actually if you pushed the caliper piston back in without opening the bleeder so the fluid could escape, the fluid is pushed back into the MC along with the contaminates in the fluid. this can lead to the rubber cups in the MC getting scared and therefore fluid bypassing and then no pedal. Or in one other case I had the fluid block the MC outlets and not letting the fluid return to the cylinder. The brakes would drag until the line was cracked at the MC. Either way a new one will fix it.
 
"my bleeding skills are on point, tube on the bleeder, see the bubbles. in a bottle with a little fluid in it.. so if it sucks it up its sucks fresh fluid. snug fitting tube on the bleeder so no air can get in via that way"
Click to expand...
I'm not so sure about your bleeding skills being on point.....
Didja seal the bleeders to the calipers? If not air will enter past the bleeder threads.
But I really don't understand what you are doing,in this quote.
In my experience, the fastest successful way to bleed brakes at home,is with a helper. And the fastest way to mess it up, is to not have a helper.
To crack a screw your way,and to crawl into the cab and start pumping,MAY NOT WORK. You will pump fluid out on the downstroke all right. But when the pedal returns, the piston will suck the same fluid back up into the caliper with, if you didn't seal the bleeder threads,mixed with air that found its way past the threads. If you seal the threads, and the bottle is lower than the bleeder, that returning fluid will STILL suck the same fluid back. The proof of this is when after 20 or 30 pumps, the fluid level in the reservoir has not changed or has not changed by much!
If you're gonna or a forced to,do it by yourself, then this is how I would recommend that you do it;
Block the pedal up, and remove a bleeder. Apply anti-seize to the bleeder threads, and reinstall it finger tight.Repeat on other side, but snug it down. Return to first side. Install and secure a long section of clear hose onto the bleeder. Take the other end and submerge it in the M/C, clamping it to the side with a clothespin or something. It is very important,obviously, that this tube not come loose and flip around. Route the tube so that it runs continuously uphill. This will now form a kindof check valve.Now go pump the pedal about ten times. Return to the bleeder, and prove the fluid is traveling up the tube.If it is; top up the reservoir, and repeat as often as necessary, until no more bubbles appear. Snug the bleeder and repeat the procedure on the other side.
But if the fluid is not traveling up the tube, this means it is probably shuttling back and forth. This is a PITA when it happens. To stop this nonsense, you will have to adjust the bleeder a lil tighter.
They make one-man bleeders to make this job easier.
There are other ways to do this, to be sure. I know a few shortcuts, but this way has always worked for me. You just have to be real careful with the loose end of the tube, around your paint job.
Alternatively, you can run the loose end of a shorter tube into a clear soda-bottle, with a bit of brake fluid in it, and the tube,again, submerged. You can secure the bottle anywhere, but the first six inches or so, out of the caliper,should be uphill.Good luck.
 
Last edited:
had a helper, i was at the caliper, the whole bottle thing with a tube is actually quite a normal procedure to make 100% sure you dont get air in the brakes. if you are not familiar with it, look it up on youtube/google. quite a lot of content.

master cilinder + bench bleeding tool is ordered and coming across the pond
 
Update: had the car up on a car-lift (buddy owns a carshop) we bled the brakes together this time.

still to the floor and light on, so it isnt that.. a good stream of oil comes out without bubbling

adjusted the rear brakes, barely needed adjustment..

couldnt find any obvious leaks.. no wet oily spots or anything

his conclusion: lets wait for the M/C to arrive, see what that does..
 
You must log in or register to reply here.
-

Back
Top