4 wheel disk low pedal blues

-
prorac1

prorac1

Well-Known Member
Joined
Sep 29, 2012
Messages
1,510
Reaction score
1,340
Location
Martin, Mi
1963 Plymouth valiant.

1974 Dodge Challenger front disks

1996 Ford 8.8 rear axle with factory disks.

1987 Dodge diplomat master cylinder. 1-1/32 bore.

Lines ran straight to hoses/calipers. No proportioning valve. Front to rear bias seems to be fine. No premature rear lock up or anything.

Here's the problem.

The brakes work really really well. But there's way to much pedal travel in my opinion.

Manual brakes. 2-3" of travel.

I'm thinking residual check valves are needed? I've plumbed them on other cars before when the master cylinder is mounted below the calipers, but didn't figure it would be a problem in this car. (With a standard master above calipers setup).

Any help is appreciated.

Thanks. Eric
 
MAster is not for 4 wheel disc brake but for Disc/ drum system.
 
THE ONLY THING(s) that affect pedal travel are:


Air in the system

Master cylinder too small in relation to stroke/ size of calipers

Calipers too large actually same as above

Some reason pads/ pistons are not "seating" on the disk and are creating more travel than normal

In the case of drum brakes,........adjusted too loose

Expanding "balooning" hoses

Prop valves, check valves, residual valves, none of that will cause more pedal stroke

What percentage of the total pedal travel are you using? Mine uses maybe 1/3 of the pedal. Mine is Linc/ Versailles factory Ford 9" rear disk, no power, and fronts are Duster/ Demon factory disk. Works fine. Has a nice "rock hard" pedal
 
67Dart273 said:
THE ONLY THING(s) that affect pedal travel are:


Air in the system

Master cylinder too small in relation to stroke/ size of calipers

Calipers too large actually same as above

Some reason pads/ pistons are not "seating" on the disk and are creating more travel than normal

In the case of drum brakes,........adjusted too loose

Expanding "balooning" hoses

Prop valves, check valves, residual valves, none of that will cause more pedal stroke

What percentage of the total pedal travel are you using? Mine uses maybe 1/3 of the pedal. Mine is Linc/ Versailles factory Ford 9" rear disk, no power, and fronts are Duster/ Demon factory disk. Works fine. Has a nice "rock hard" pedal
Click to expand...


Wouldn't to much fluid returning to the master cause a low pedal also?

That was my thoughts behind the residual valves.

I can't pump the pedal up. So I don't believe its air. But I am planing on bleeding again. Now that we've been pumping the brakes for a couple of months.

Yeah, I'd say it uses just over a third of the pedal travel until it's rock hard.

But even than it feels a little spongey.

Just not what I'm use to. Usually I can get a short travel and a rock hard pedal.

Thanks. Eric
 
67Dart273 said:
THE ONLY THING(s) that affect pedal travel are:


Air in the system

Master cylinder too small in relation to stroke/ size of calipers

Calipers too large actually same as above

Some reason pads/ pistons are not "seating" on the disk and are creating more travel than normal

In the case of drum brakes,........adjusted too loose

Expanding "balooning" hoses

Prop valves, check valves, residual valves, none of that will cause more pedal stroke

What percentage of the total pedal travel are you using? Mine uses maybe 1/3 of the pedal. Mine is Linc/ Versailles factory Ford 9" rear disk, no power, and fronts are Duster/ Demon factory disk. Works fine. Has a nice "rock hard" pedal
Click to expand...

And if I may ask, what bore is your master? I may be off on the bore. That would cause this.

Thanks. Eric
 
I don't know maybe you can look it up. It is the bone stock 67 Dart drum master that was in the car. I merely punctured the residual valves. Hell I wasn't sure it'd work LOL. Worked GREAT!!
 
67Dart273 said:
I don't know maybe you can look it up. It is the bone stock 67 Dart drum master that was in the car. I merely punctured the residual valves. Hell I wasn't sure it'd work LOL. Worked GREAT!!
Click to expand...

Appears to be 1" bore
 
If the rear calipers are equiped with an internal emergency brake (ratchet screw) behind the Piston, the emergency lever may need to be cycled several times to get the pads/pistons into contact with the disc.


 
inertia said:
If the rear calipers are equiped with an internal emergency brake (ratchet screw) behind the Piston, the emergency lever may need to be cycled several times to get the pads/pistons into contact with the disc.


Click to expand...



Explorer axles have drum style parking brake. I thought about that too.

Thanks. Eric
 
Actually, there is one more thing that can cause long pedal travel. If the pedal pushrod is adjustable and is adjusted too short, that will do it.
 
Drum brake pedal geometry. The original self energizing drum brakes needed minimal brake pedal travel/pressure to supply enough leverage to stop the car when in proper adjustment. The brake system is also a "system" . Just like saying I got this cam and this carb and this manifold and these heads and my car doesn't run right. It has to be compatible and matched. Frankenstein brake systems usually work just like Frankensteins' Monster. Slow, awkward and not always predictable. Try to use a complete matching brake setup. Same car front & rear brakes and matching brake pedal/master cylinder setup. Otherwise you will be guessing which bore diameter master or calipers will I need etc... Standard 4 wheel disc will never feel as hard as a disc/drum set up. You can't really adjust pedal height with a disc caliper only a brake shoe/drum. I may FOS of course also.
 
MoparLeo said:
Drum brake pedal geometry. The original self energizing drum brakes needed minimal brake pedal travel/pressure to supply enough leverage to stop the car when in proper adjustment. The brake system is also a "system" . Just like saying I got this cam and this carb and this manifold and these heads and my car doesn't run right. It has to be compatible and matched. Frankenstein brake systems usually work just like Frankensteins' Monster. Slow, awkward and not always predictable. Try to use a complete matching brake setup. Same car front & rear brakes and matching brake pedal/master cylinder setup. Otherwise you will be guessing which bore diameter master or calipers will I need etc... Standard 4 wheel disc will never feel as hard as a disc/drum set up. You can't really adjust pedal height with a disc caliper only a brake shoe/drum. I may FOS of course also.
Click to expand...
> his post said 1 3/16'' master cyl., needs 15/16 to start with. I have an 87 diplomat master cyl. on my 68 fastback, with willwood discs on front , and chevy type discs on a dana 60 in back, no problems what so ever. I did make my original brake pushrod longer the same exact length of the offset master cyl. mount .---------
 
RustyRatRod said:
Actually, there is one more thing that can cause long pedal travel. If the pedal pushrod is adjustable and is adjusted too short, that will do it.
Click to expand...

famous bob said:
> his post said 1 3/16'' master cyl., needs 15/16 to start with. I have an 87 diplomat master cyl. on my 68 fastback, with willwood discs on front , and chevy type discs on a dana 60 in back, no problems what so ever. I did make my original brake pushrod longer the same exact length of the offset master cyl. mount .---------
Click to expand...

I am running the same master cylinder you are Bob. 1987 dodge diplomat. 1-1/32 bore. We did not lengthen the pushrod. I never thought about it. I believe we have an extra push rod, I think we're going to try that.

Bob, are you running any proportioning valve or residual pressure valves in your system? Thanks. Eric
 
Check what you have first. When I replaced the master cylinder on my Ford truck, there were several options available. I went with the Bendix casting, because that's the model that came on the truck. Because of machining tolerances, I had to shorten the pushrod, because it had pressure applied to the front brakes right off the bat. Hopefully, it's that simple. If you're like me, probably not. LOL I just got lucky with mine.
 
I would try to get a really good bench bleed on the master cylinder . took me 3 times to get right . (Par for the course ) Then go on to the good advise previously posted .
 
prorac1 said:
I am running the same master cylinder you are Bob. 1987 dodge diplomat. 1-1/32 bore. We did not lengthen the pushrod. I never thought about it. I believe we have an extra push rod, I think we're going to try that.

Bob, are you running any proportioning valve or residual pressure valves in your system? Thanks. Eric
Click to expand...

You can easily check the push rod. If you can't get "up in there" and feel it, just try wiggling the pedal gently "up/ down" the pedal/ rod should be "sitting" on the master piston with no slack. You should be able to "pick up" the pedal and "sit it down" sort of and feel the slack. Try to determine if the pedal is up far enough to pretty much fully stroke the master, although you don't want to bottom it, either.

Unfortunately, checking what the master will stroke might just require tearing it down and draining it
 
67Dart273 said:
You can easily check the push rod. If you can't get "up in there" and feel it, just try wiggling the pedal gently "up/ down" the pedal/ rod should be "sitting" on the master piston with no slack. You should be able to "pick up" the pedal and "sit it down" sort of and feel the slack. Try to determine if the pedal is up far enough to pretty much fully stroke the master, although you don't want to bottom it, either.

Unfortunately, checking what the master will stroke might just require tearing it down and draining it
Click to expand...

I'll crawl my scrawny *** under there and check it next time I get over there.

Now that it's been together and worked for a while I'd like to bleed the entire system one more time.

Thanks. Eric
 
prorac1 said:
I am running the same master cylinder you are Bob. 1987 dodge diplomat. 1-1/32 bore. We did not lengthen the pushrod. I never thought about it. I believe we have an extra push rod, I think we're going to try that.

Bob, are you running any proportioning valve or residual pressure valves in your system? Thanks. Eric
Click to expand...
Mine is a 15/16" master cyl, pedal feel is about twice what new cars disc brakes are. ???????????
No special valves , just the stock splitter from the drum brakes.
Are u running an offset plate on ur master cyl. ? If u are and its 1/2" , that's how much longer ur pushrod needs to be ---------
I cut mine, threaded it, screwed it together w/ a threaded rod coupling, adjusted the length, welded it , then turned it down to look like a car part, and not to rub anywhere.
 
Last edited:
proportioning vqalvesw, just the orig. brass splitter from the stock brakes.
prorac1 said:
Do you mean you have to push the pedal twice as far as a new car?
Click to expand...
No , just about twice the pressure feel of a new set of power brakes. Fairly happy w/ it !!
Don't have to use them a whole lot tho, when I let off, the engine slows it down for a non hurried stop, but work very well when I need to stop. Pedal travel is about normal ----------1 1/2' or maybe a hair less ???
 
famous bob said:
Mine is a 15/16" master cyl, pedal feel is about twice what new cars disc brakes are. ???????????
No special valves , just the stock splitter from the drum brakes.
Are u running an offset plate on ur master cyl. ? If u are and its 1/2" , that's how much longer ur pushrod needs to be ---------
I cut mine, threaded it, screwed it together w/ a threaded rod coupling, adjusted the length, welded it , then turned it down to look like a car part, and not to rub anywhere.
Click to expand...

No offset adapter, just the 1/4" thick 4 hole to 2 hole adapter.

I thought you had an 87 diplomat master cylinder? All of my research shows that as a 1-1/32?

Thanks again for all the help. Eric
 
prorac1 said:
No offset adapter, just the 1/4" thick 4 hole to 2 hole adapter.

I thought you had an 87 diplomat master cylinder? All of my research shows that as a 1-1/32?

Thanks again for all the help. Eric
Click to expand...

If 1/4' then u need to lengthen the rod 1/4'' or you pedal might be down too far to start with , in my opinion,----never seen one that thin.
When I bled the master cyl. before installation I measured it out of curiosity, was less than 1"'. All I know is I`m happy with it !
I took a members advise on the master cyl. when I completly redid my brake system, lines, discs and everything, and an 87 diplomat master cyl. is what I asked for at the local o`riellies. ????????????
 
Here’s a picture of our adapter.


E6EC86DD-4574-45DA-BA07-91B0D6217BD9.jpeg
 
You must log in or register to reply here.
-

Back
Top