47-55.1 Brake lockup issue

[Jbrenyo]

My son and I are almost complete with the rebuild of his truck (1953 Chevy truck on an S10 frame with an LS1 and Tremec 6 speed) and have encountered a perplexing problem: the rear disc brakes do not release. At first we thought that the calipers that came with the rear end were the culprit because they were old and used, so we replaced the entire rear brake platform: rotors, calipers and pads. They still refuse to release, so we replaced the hoses to the calipers. Same problem. The only things left that we can think of (working backwards) is the proportioning valve, the master cylinder, and the brake booster. Neither of us has ever messed with any of that stuff so before we do I thought I would ask the experts in this forum what they would do next. All opinions are welcome. Thanks!

[1project2many]

Well, I suppose that would be a perplexing problem. Rather than throwing parts at the truck to see what happens I would suggest trying a few steps to diagnose the issue.

First, are the rotors free to turn after the calipers are first installed? Do the calipers lock after applying the brakes a couple of times? That indicates the problem is likely a hydraulic issue.

Testing to see if the hydraulics are keeping the calipers tight is easy. Open the bleeder or loosen the brake line to see if the calipers release. If the calipers stay locked there's a problem with caliper slides or with your mounting/pad thickness/rotor choice combination.

If releasing hydraulic pressure releases the rotors then it's easy to move from the calipers forward to find the problem. Apply and release the brakes so the calipers lock then disconnect the steel line from the top of the flex hose. Do the calipers stay locked? The problem is likely in the flex hose. Do the Calipers release? Move to the next point and try again. If you get all the way to the master and you can't find the problem, there's a good chance it's related to the master.

[mongocanfly]

Make sure your pedal is releasing the master cylinder push rod...when the pedal comes back it should release all pressure on the system...the master pushrod should be all the way back

[Volfandt]

I had a similar problem on a front disc brake conversion I did on a 49 3100.
What I found was the stock OEM calipers had a little cast boss on the edge that contacted the aftermarket mounting brkt and the brkt wasn't notched to allow the caliper to retract. 1st time I applied the brakes in testing the wheels would lock up.
A little fab'ing with a grinder and dremel produced a notch in the brkt that allowed full free movement of the caliper and good brakes.
Good luck
Dave

[Jbrenyo]

So after changing both rear hoses and bleeding the brakes for the umpteenth time the non-retracting piston problem seems to have been resolved. Thanks to everyone that took the time to respond.

Question: how do we know we have the correct proportioning valve for the application? The one currently in the truck is a non-adjustable GM one.

[MiraclePieCo]

Originally Posted by Jbrenyo

So after changing both rear hoses and bleeding the brakes for the umpteenth time the non-retracting piston problem seems to have been resolved.

Sounds like your hoses were swollen shut, acting like a one-way valve allowing pedal pressure in but not out.

Originally Posted by Jbrenyo

Question: how do we know we have the correct proportioning valve for the application? The one currently in the truck is a non-adjustable GM one.

Take it to a gravel parking lot and perform a skid test. Note if the fronts or rears are locking first. My guess is you're okay; the common mid-size GM units seem to be pretty close to the weights for these trucks.

[mr48chev]

On the proportioning valve normally you don't need an adjustable one unless you are having issues with the rears locking up before the fronts and causing the rear to go sideways.

It doesn't have to be a gravel lot but you want lots of room when you do your braking tests. Maybe the hobby lobby lot on Sunday morning or an industrial area lot. It might even be best to try it on more than one surface and a wet surface. If it stops pretty straight and feels right life is good If it gets a bit or more than a bit wonky on you then it is time to figure out how to make it better.

[dsraven]

so, just in case the issue comes back or someone else is having the same issue and has some questions
-ensure there is a pedal return spring and a stopper for the top pedal height so the pedal stops at the same place every time
-ensure the pedal is free moving-remove the linkages or pushrod to the rest of the system and ensure the pedal moves very easily from the floor to the top of the pedal travel where it stops against a stopper of some sort
-ensure the pedal linkages to the booster/master cylinder are not worn or binding on anything even with a full throttle engine torquing up or decelerating
-ensure the booster/master cylinder linkage has some free play with the pedal at the top against the stopper. push the pedal down and let it return on it's own, check to see it is touching the stopper, then check the free play at the booster/master cylinder. there needs to be some free play at the booster/master cylinder end
-check to ensure there is free play at the master cylinder from the booster pushrod (if power brakes). a quick check for this is to remove the master cylinder fill cap (if it is not a remote reservoir) and have a buddy step on the brakes while you watch the fluid in the res. there should be a quick "fountain" in the fluid when the brake pedal is first stepped on. that is because the pistons in the master cylinder are behind the ports that allow fluid to return to the reservoir (as the fluid heats up and expands from use and/or to fill the cylinder as fluid is used in the wheel cylinders or calipers as the brakes wear). there should be that little fountain as the piston moves forward and then the piston passes the fill ports and starts to move fluid down the brake lines to the wheel cylinders or calipers. no fountain present could indicate the pushrod is not letting the master cylinder pistons return fully when the brakes are released and this will keep the brakes slightly applied. this may be worse when the brakes heat up and the fluid expands in the lines and has nowhere to go except to apply the brakes more as the heat expands the fluid more.
-check to ensure there is no pressure at the bleeder screws when the brakes are released. a slight pressure is normal on drum brakes because the residual valve in the master cylinder keeps a couple of pounds pressure in the lines for the wheel cylinders to keep their cups tight against the wheel cylinder bores and create the required seal.
-check to ensure you have the right master cylinder for your application-disc/drum or disc/disc-a drum brake master cylinder may have the residual valve pressed into the port on the master and if that was used on a disc/disc set up the valve needs to be removed or the proper master cylinder for your application should be used.
-if doing the bleeder screw pressure test and there is pressure at the bleeder screw, close the bleeder and re-apply the brakes and release. then go to the next fitting upstream of the bleeder screw (usually a rubber hose on a disc brake application) and loosen the line connection to see if there is pressure there-ahead of the bleeder screw- if so the problem would appear to be further up stream of that fitting. if not then the problem would appear to be in the actual steel line or rubber line-it is working like a check valve as mentioned above. sometimes it is a pinched steel line where something has bumped against it,especially if using the stock steel lines that came on the donor vehicle. on a rubber line is can be a deformed line from time spent sitting in a parts box or on a hook at the parts store, a bad crimp during the manufacture process of a new line or even an old rubber line that is disintegrating on the inside causing a flap like one way valve. have someone step on the brakes really hard and watch the rubber line for excessive expansion
if using the stock s10 prop valve you will likely be fine because the truck probably has about the same weight ratio front to rear. that was before the v8 was dumped in though. a parking lot test, as mentioned above by the pros, is a great idea. wet and dry-empty parking lot.take video so you can slo mo and see whats happening. an adjustable prop valve is also available but when set up will likely not need any more adjustment so mount it accordingly if going that route.

[Matt_50]

Just saved this thread. Getting a custom line made soon and hope I don't have issues.

[Jbrenyo]

Thanks to everyone for all their input.