Rear discs doing all the work and very hot

876 Views 24 Replies 10 Participants Last post by Ford blue blood, 10 d ago

Skooch62

Discussion Starter · 21 d ago

I’ve got Willwood discs on all 4 wheels , with non-adjustable proportioning valve and residual valves on fronts and backs. Basic master cylinder Under the floor. I’ve replaced all seals in the calipers and have bled many times. The issue is the fronts are only slightly braking and the rears are getting very hot. 230 degrees while the fronts are at 80 . Someone said get an adjustable proportioning valve and another said replace the master cylinder. Please help !!!

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Hotrod46

· #2 · 21 d ago (Edited)

If the proportioning valve you have is a stock type (as installed by OEM's), then it could have an internal cutoff built in.

This is the type:

www.speedwaymotors.com

GM Disc/Disc Proportioning Valve for 1/2-20, 9/16-18 Outlets

These proportioning valves are similar to what GM used on many cars in the 70s. They are non-adjustable and may require an adjustable valve in the rear brakes (if needed). Block incorporates metering (for front brakes) and proportioning (for rear brakes) into one unit. For use with four-wheel...

www.speedwaymotors.com

The cutoff is a piston that is free to float between the front and rear circuits. It's purpose is to cutoff either end of the brake circuit if a leak develops on that end. This can be a big problem when bleeding brakes since the valve will move toward the end that is being bled and shut it off or severely restrict the flow. This same piston will also typically operate a switch that activates a brake system warning light. There are tools available to lock the piston in place while bleeding. The tools are used by removing the warning switch and screwing in the tool where the switch goes.

It is possible that while bleeding the brakes on your car that the cutoff has shifted to the front circuit.

This is the tool:

https://www.summitracing.com/parts/sum-760158?seid=srese1&ppckw=pmax-tools&gclid=Cj0KCQjwsIejBhDOARIsANYqkD34M4UtF2a-SXDYIpLmZtq4qtuHtNxF2CDi2OMpc5MNT1xJgaGh2fMaAj8uEALw_wcB

This COULD be your problem, but is just a possible cause. Just something to check.

You can sometiimes recenter the piston by cracking a bleeder on the opposite end of the car and gently pressing the brake pedal. Doesn't always work and the piston CAN get stuck.

Here is an internal diagram of a typical proportioning valve with this type of cutoff:

https://qph.cf2.quoracdn.net/main-qimg-32a9981a5f7ea75f044cf67a8d3eac11

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Reactions: 1

39 master

· #3 · 21 d ago

You didn't say what M/C you have but I've had problems with a 'Vette (GM) unit where the pedal was pushed before the system was filled and one piston in the m/c stuck at the far end. Doesn't sound like your problem because the front piston goes to the back. When bleeding is the front really squirting fluid or just a dribble?

Skooch62

Discussion Starter · #4 · 20 d ago

39 master said:
You didn't say what M/C you have but I've had problems with a 'Vette (GM) unit where the pedal was pushed before the system was filled and one piston in the m/c stuck at the far end. Doesn't sound like your problem because the front piston goes to the back. When bleeding is the front really squirting fluid or just a dribble?

When bleeding the fronts , fluid came flowing out .

39 master

· #5 · 20 d ago

When you say "flowing" I assume it's somewhere between dribble and squirt. Sounds like the front isn't getting the pressure so I would break the line at the prop. valve and see the flow there. If it's still not robust break the line by the residual valve. In other words keep going back until you get good squirt. May go clear back to the M/C but necessary to define where the problem is. Bit messy too.

Ford blue blood

· #6 · 20 d ago

What is the master from? Is it a 4 wheel disc master?

Skooch62

Discussion Starter · #7 · 20 d ago

Ford blue blood said:
What is the master from? Is it a 4 wheel disc master?

Good question. I have no idea where it’s from. Is there anyway of telling if it’s a 4 wheel by looking ?

Reactions: 1

Skooch62

Discussion Starter · #8 · 20 d ago

Skooch62 said:
Good question. I have no idea where it’s from. Is there anyway of telling if it’s a 4 wheel by looking ?

it’s very similar to this one if that helps

Gas Metal Fashion accessory Event Auto part

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Ford blue blood

· #9 · 20 d ago

Disc/drum masters have a residual valve in the port for the drums. Pull the master off and look inside the port, if there is a brass/rubber thing in there put a screw in it and pull it out. It sounds like, from your description, that the rear brakes have pressure on them.

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Hotrod46

· #10 · 20 d ago

Most of the time a disc/drum MC will have reservoirs of different sizes, like the one in your picture. A large one for the discs and a smaller one for the drums. The drum side will typically, but not always, have residual valve built in to the outlet port. Stock disc brake MC's rarely have any residual valves in them. I have seen some aftermarket MC's that were advertised as disc/drum that actually look like a disc/disc MC. Can't comment on them.

Skooch62

Discussion Starter · #11 · 20 d ago

Ford blue blood said:
Disc/drum masters have a residual valve in the port for the drums. Pull the master off and look inside the port, if there is a brass/rubber thing in there put a screw in it and pull it out. It sounds like, from your description, that the rear brakes have pressure on them.

It does have an inline residual valve going to the back brakes and one each going to the fronts. Could these be the issue ?

Ford blue blood

· #12 · 20 d ago

Conventional build protocol says if the master is below the brake pistons irrespective of disc/disc or disc/drum that residual valves be inserted in the lines feeding the pistons. 2psi for disc and 10psi for the drums. The residual valve I was referring to is in the master. The residual valves in a system where the master is lower then the pistons is to prevent drain back into the master reservoirs when at rest.

Your system should have both the front and rear feed lines with 2psi valves only. If that is a disc/drum master you have identified the issue as it will most likely have the residual valve in the output port.

Personally I don't like have the residual valves in line, just an opportunity for a problem down the line. My cars have the fill reservoirs up on the fire wall to let mean ol mr gravity take care of the drain back issue.

Reactions: 1

RODSRUS

· #13 · 20 d ago

This is an aftermarket Ford style master cylinder or from a Mustang/Granada/Fairlane late 70's to early 80's. What is odd about this problem is that the front and rear master cylinder pistons are not rigidly connected. The front piston must move before the spring between the pistons lets the rear piston move. It makes it more difficult to bench bleed the master cylinder. The problem I had was the rear brakes didn't do hardly any thing (also 4whl disks) until I went to a smaller diameter master cylinder. So I'm baffled how the rear brakes can be doing all the work.

Ford blue blood

· #14 · 19 d ago

IMHO you need to get a know 4 wheel disc brake master cylinder. Check the in line residual valves, one for the front disc and one for the rear disc that are 2 psi, that they are installed correctly and in the line before any distribution blocks. The fact that the rears are heating up as high as they are says there is residual pressure holding the pads against the disc.

'48 Austin

· #15 · 19 d ago

Easy way to check if master cylinder has built in residual pressure valves is to remove brake lines at MC and attempt to insert a straightened paper clip. If it easily inserts all the way into the main bore of the MC, it has no residual pressure valves.

RODSRUS

· #16 · 19 d ago

The large master cylinder reservoir is plumbed to the front brakes isn't it?

blwn31

· #17 · 19 d ago

Can you get a pic of the master?

Keith

RODSRUS

· #18 · 19 d ago

This Ford style master cylinder (see the previous pic) is either aftermarket or from a around 80's Mustang/Fairlane/Granada (disk/drum). If aftermarket then it is likely Speedway 910-31420 which doesn't have a residual valve. Although the Speedway words say disk/drum, in the Q/A section it says they are for disk/disk also. Because with this Ford style master cylinder the front brake piston must move some before the rear brake piston can move I'd look down stream for the problem (Proportioning valve or residual valve).

RODSRUS

· #19 · 19 d ago

I have one of these master cylinders on my work bench right now and the front brake piston and spring assembly comes right out but the rear brake piston isn't coming out even with some air pressure. Maybe the problem is as simple as a stuck rear brake piston. I'm not sure what the purpose of having this non physically connected two piston design is and I can't see what retracts the rear brake piston when the front piston retracts except line pressure. The aftermarket pedal/master cylinder assemblies (which is what you have if it is for your '32) use either this Ford style or Corvette style master cylinders.

blwn31

· #20 · 19 d ago

Just a dumb question, are your brake lines plumbed to the correct ports on the master cylinder?

Keith

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