Front brakes locking from residual pressure

[ttkelly]

I just replaced a brakeline and master cylinder and my front brakes are dragging. I have the chevy front brake conversion with a proprtioning valve. In diagnosing the problem when I open the bleed screw on my caliper the wheel spins freely thus the calipers seem to be functioning fine. After closing the bleed and reapplying the brake the wheel sticks again. Is there something on my proportioning valve I need to adjust? Photo is attached below of my proportioning valve. Any ideas or suggestions would be appreciated!

 

[miikee73]

Just a shot in the dark... is the master cylinder for disc or drum ? If drum than there is a puessure valve in the rear {front brake] res. that needs to be removed.

 

[ttkelly]

Just a shot in the dark... is the master cylinder for disc or drum ? If drum than there is a puessure valve in the rear {front brake] res. that needs to be removed.

Thanks for the post... This is my master cylinder. I think it is the stock replacement. Where would the pressure valve be on this unit?

 

[miikee73]

the rear port supplies the front brake,pull the rear line off and there should be a rubber cup,spring and ball in there ,just remove them and hook it back up and bleed system.

 

[miikee73]

Front drums to disc......some valve/check ball to remove? read this thread under bronco tech....search..master cylinder,It will give you more info.
http://classicbroncos.com/forums/showthread.php?t=139861&highlight=master+cylinder

 

[broncnaz]

I would also take a look at your prop valve the pin sticking out the front is not a good sign. They are not supposed to stay stuck out. you could try pushing it back in see if that helps any but there may be something wrong with your valve.

 

[Apogee]

You should also confirm that you have a small amount (.015" to .030") of free-play between the pushrod and the master cylinder piston. If the piston isn't being allowed to fully retract, it won't allow the pressure to bleed off and return fluid back into the MC reservoir.

 

[cannonnum4]

Just don't use them!

 

[SDlivin]

You should also confirm that you have a small amount (.015" to .030") of free-play between the pushrod and the master cylinder piston. If the piston isn't being allowed to fully retract, it won't allow the pressure to bleed off and return fluid back into the MC reservoir.

Sounds like a couple possible options here, but the above was the challenge I had when I replaced my MC.

I checked everything else, but the front brakes would still stay clamped on once brake pedal pushed, if I let it sit a couple hours/over night it would release.

I added 2 spacer washers (1/16th - 1/8") between my MC and Booster and this fixed the problem of the pushrod.

 

[fordtrucks4ever]

Doesnt your booster rod have the adjustable nut on end to set at correct length for proper clearance to M/C piston?

 

[surfer-b]

Doesnt your booster rod have the adjustable nut on end to set at correct length for proper clearance to M/C piston?

All the ones I have seen do.

 

[SDlivin]

Doesnt your booster rod have the adjustable nut on end to set at correct length for proper clearance to M/C piston?

Yes, but it didn't adjust enough... I ended up just needing 1 more washers space to get it to work and release the front calipers.

 

[Apogee]

The piston seals inside the master cylinder need to retract far enough to clear the compensator ports, otherwise fluid/pressure can't return to the MC reservoir.

Another possibility would be a failing center hose from the frame down to the axle. As the rubber hoses age and fail, they can split and swell internally to the point that they can actually maintain some residual pressure in the calipers. It wouldn't take much to not be able to spin the calipers by hand...but it takes substantially more pressure to lock them up with the vehicle's weight on the tires. Dragging calipers can overheat, seize and lead to other issues worse than accelerated pad wear.

 

[VT_Don]

I had the exact same condition on another Ford brake system overhaul. As suggested, check the push rod adjustment. Take the lid off the MC and have someone work the brake peddle. If the fluid is fresh and clear, should be able to clearly see thru the port at the bottom of the reservoir. With no peddle pressure, the cylinder should retract enough to not block fluid return to the port. In my application, there is a thin, white plastic spacer between the booster and MC. No parts manual makes any reference to it, so it had been left out. You don't want to know how long it took me to sort it out.

 

[DirtDonk]

the rear port supplies the front brake,pull the rear line off and there should be a rubber cup,spring and ball in there ,just remove them and hook it back up and bleed system.

What he said, but the stuff will be behind the brass cone that you first see, so you have to remove that first.
While some may just pop right out, most of us have to pull on them pretty good. A sheet-metal screw of just the right size as to thread into the hole in the tip of the cone, but not so big as to distort it, can be used to pull it out.
Once it's out, that other stuff mikee mentioned will be visible if it's there.

Paul

 

[ttkelly]

What he said, but the stuff will be behind the brass cone that you first see, so you have to remove that first.
While some may just pop right out, most of us have to pull on them pretty good. A sheet-metal screw of just the right size as to thread into the hole in the tip of the cone, but not so big as to distort it, can be used to pull it out.
Once it's out, that other stuff mikee mentioned will be visible if it's there.

Paul

You were right about it being stubborn! I need to find that one well fitting screw! I already took a bit of the brass off with my first attempt. The cone is still in pretty good shape, however if I were to mess it up more is this a thing I could easily replace? Or maybe the question is if it is even needed? If I didn't have to worry about the cone I could be done now. Thanks everyone for the advice thus far... It fells like I am in the home stretch.

 

[ttkelly]

I had the exact same condition on another Ford brake system overhaul. As suggested, check the push rod adjustment. Take the lid off the MC and have someone work the brake peddle. If the fluid is fresh and clear, should be able to clearly see thru the port at the bottom of the reservoir. With no peddle pressure, the cylinder should retract enough to not block fluid return to the port. In my application, there is a thin, white plastic spacer between the booster and MC. No parts manual makes any reference to it, so it had been left out. You don't want to know how long it took me to sort it out.

All seems clear on the rod length front. The length is the same size as the replacement and I can see through the port when the pedal is not pressed.

 

[DirtDonk]

As long as the cone is not totally buggered, you should be able to re-use it. It's definitely necessary for the line to seal against, but it's just soft enough that if you've got a little distortion, tightening the tube back in the port should re-form it to seal correctly.
At least that's what mine did, as the tip of the cone was a little distorted when I did mine too.

Good luck.

Paul