Brake pedal goes to floor with engine running

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74dusterman

74dusterman

74dusterman
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It's not an A body but I was hoping you guys could help. I am having troubles with my truck. The running gear is from a 1977 W250 and the body is a 1986 D150. The brakes have not been right since I bought it this summer. I replaced both front calipers because they were sticking. I bled them properly and still have problems. When I pump the brakes without the engine running the pedal feels good and barely goes down. Start the engine and the pedal goes to the floor and brakes are poor. I tried changing the master cylinder with another used one I had and the same problem. I checked the booster according to the manual and everything checks out ok. No vaccuum leaks. No fluid leaks. The rear brakes are adjusted properly. I have ran 2 quarts thru the lines to make sure I bled them good and still the pedal goes to the floor when I start the engine. The brakes grab enough to stop but not enough to stop in an emergency. I have to push the pedal as hard as I can to get it to stop quickly. I am talking about bracing up against the back of the seat and giving it all I got. Has anyone had a similar problem? I am thinking it is the master cylinder but wanted to see if someone else had the same issue before I order parts. Any help would be appreciated.
 
The description above notes that the brakes are good without the engine running WHEN THE BRAKES ARE PUMPED UP. That is the problem right there; there is still air in one half or both halves of the system. The pedal should not pump up at all under any circumstances. BTW, the pedal going down SOME when starting the engine with a power brake booster is normal, but the booster is making the air-in-the-system problem worse.

I would suspect that the first MC was not bench bled, or the wheel cylinder and caliper bleeding technique is not right. Same for the 2nd MC. BTW, I would never re-use an MC unless it was absolutley known good, and any installed MC has to be bench bled prior to putting in the vehicle.

One other thing to consider is that the MC be right for the bakes front and rear. I assume you have rear durms and the MC for that will have check valve to keep the fluid from the rear wheel cylinders being pushed back by the springs.

And, make sure the fornt and rear lines to the MC are connected right; the fronts should go to the larger MC reservoir.
 
Front are disc rear are drum. Bleeding technique is the good old 2 person pump them up, hold pedal, open bleeder, pedal goes to the floor, close bleeder, repeat the process. Starting farthest away from mc. I don't know how there could be air in the lines because I ran a quart of fluid out the rear and a quart out the front. I ordered a new mc this week. I will see if that solves it.
 
Did you do the bleeding with the m/c cover or caps off?
I ran into this one time with the same results and was about to shoot the truck when i tried it with the cover off and it worked.
Gawd did i feel stupid.........
Other than that, i would guess the booster is toast or the one way valve on the top is stuck or faulty.
With the amount of pedal effort you say it has, it sounds like the booster is your problem.
When you pull the little valve out of the top of the booster, it should hiss with the vacuum escaping.
Try both.
 
DON'T pump the brakes with the top/caps off the M/C,, as it may spray brake fluid/(paint remover) all over your engine compartment and perhaps fender and cowl.. !!

If you do accidentally get br. fluid on paint,, rinse IMMEDIATELY with water,,

Ask me how I know first hand... lol .. and had a few apprentices too...

hope it helps..
 
No problem with the cap off, but you have to do it REEEEAL slow; I do it with the caps on for that issue.

To the OP, sometimes the MC can get too full of air if drain and then pumped with no fluid in the reserviors, and you have to bench bleed it again. Not very common in my experience but have had this happen once or twice in 40 years.

One other possibility of the 'pumping up', when the engine is off is that there is residual vacuum in the booster that makes the pedal go down further but you bleed that residual vacuum off with a few pumps. It that is the case, then probably the MC. Sounds like you are on that path now. I bleed the same way, BTW.

Any way the calipers on front are reversed with the bleeders pointing down? Just checkin'....
 
I had the caps on the mc. I made the mistake of pumping the brake pedal once and shot a bunch of fluid out. I didn't try pulling the hose off the booster to see if it hisses. I checked the booster according to the manual and it checked out good without any leaks but I will try pulling the hose off and see what happens. The calipers are installed correctly. That is one of the first things I checked. I made sure the mc did not get too low when bleeding so it didn't get air and I had to start over. With the engine off and after a couple pumps the booster is empty and the pedal barely goes down and feels firm. As soon as I start it the pedal will go to the floor and there isn't much there. I have another booster but it is used. Is there any way to bench test it before changing it? They are not that fun to change because it you need 4 elbows and 2 thumbs on one hand to get at the bolts. My new mc will be here tomorrow so It would be a good time to change the booster too if I knew the other one is good. Thanks for the support so far. I appreciate it.
 
What kind of cam is in the engine? Maybe for grins put a vacuum guage on it and see what kind of vacuum its pulling. Went thru this with a small block with a decent cam.
 
The engine is a 383 out of a car. I believe it has never been rebuilt. It seems pretty tired and doesn't have a lot of power. I had a vaccuum gauge hooked up to the carb when I was trying to adjust the idle mixture screws and it was pulling around 15. I pulled the hose off the brake booster when it was running and there was a lot of vaccuum there. That is one way I was told to check the booster for leaks. If the engine doesn't change idle when it is unhooked and go back to normal when it is hooked back up then the booster is bad.
 
Are you getting vacuum to the booster? Check your vacuum source on the engine, next check your vacuum at the entry point to the booster. Could be the hose collapsing. If all good to there, look at the booster. I've seen a m/c leak fluid into the booster. Goofs up the function of the booster.
 
This past weekend I finally got a chance to work on the brakes. I changed the mastercylinder, bought a new one. I bench bled it according to instructions. Installed it and bled all 4 corners following the correct sequence and using a mighty vac bleeder. I checked the vacuum to the booster, good vacuum. Booster holds vacuum after truck is shut off. Rapid pumping does not make the engine stumble as far as I can tell. Brakes still are not very good. Rear brakes are adjusted properly. Pedal goes way down to the floor. If I had to stop in emergency situation I would have to push pedal with extreme force. MC is for W250 with 3500lb front axle and so are the new calipers. Anybody have an ideas? Air in lines somehow? Booster junk? This thing is kicking my a$$ right now. Thanks for the help guys.
 
You can check a booster off the car. Connect it to vacuum (engine running), push a rod in the input side and see if it follows your motion.

If the pedal goes way down, you have compressibility in the system somewhere, booster or not. That could be an air bubble (most common) or bulging rubber hoses. A leak can also make the pedal slowly sink (and suck in air when released), but brake fluid evaporates slowly, so unlikely you wouldn't see a leak, plus the reservoir level would drop. Most commonly, the problem is not bleeding the MC well. Look at videos on youtube. You need ~50 full strokes to get all the air bubbles out. Air in calipers is the next common problem, especially if installed w/o the bleeds at the top.
 
Plug mc ports and apply brakes. If pedal goes anywhere, its your mc. Plug rear port of distribution block, same test to isolate circuit. Gravity bleed works well, just dont let reservoir run dry. We had a 64 300, brakes sucked. Turned out to be rear adjusters, no where close. Tighten, pump and tighten again until you feel it dragging. This was a 4 drum car.
 
I would jack up the rear wheels, and adjust the brakes till they jammed the wheels tight,,
if the pedal was no better then,, I'd have a friend pump the brakes twice and hold,, I'd then take the M/C cap off, and have the pedal released,.. which-ever system (fr or rear)shoots fluid back,, I'd bleed that system.. jmo..
 
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