Front brakes still locked up

[Rough75C20]

Hello all,

Having an issue with front brakes getting locked up after pushing the pedal down a couple times. The pressure won’t release. I replaced the calipers and the rubber lines. Already but the problem remains. I’ve also blocked the proportioning valve with a tool that keeps the valve in the middle for bleeding the system so the valve shouldn’t be moving and blocking the front for the pressure to release. Any ideas? Got new calipers from oriley and there aren’t any leaks in the system.

[Mike81K10]

Even if you used the tool to block it, there is still a chance the tool was used on a bad proportioning valve.

[Rough75C20]

Is the proportioning valve something that goes bad normally and could cause this issue? I would think blocking the valve would allow fluid to easily flow through the system.

[bd]

Do the discs release if you crack the line to the front brakes at the master cylinder?

[Rough75C20]

I will check this. Didn’t try that yet. Will get back asap.

[Rough75C20]

Yes, when cracking the front line at the master cylinder brake fluid comes out obviously, but it also relieved the pressure in the front lines and the wheels speed freely. Bad master cylinder?

[VileZambonie]

Sounds like you need a master cylinder.

[bd]

I agree with this ^^^^^.

Still, ensure the brake pedal is fully returning and not hanging up.  If the pedal moves freely, build pressure again by applying the brakes as before.  Assuming the brakes are locked up again, remove the M/C mounting nuts and slide the M/C forward, away from the power brake booster.  If the brakes release simply by sliding the M/C forward, replace the power booster.  Otherwise, bench bleed a replacement M/C and perform a COMPLETE AND THOROUGH FLUID FLUSH, FRONT AND REAR, USING FRESH BRAKE FLUID FROM A PREVIOUSLY UNOPENED CONTAINER.

Do you know the truck's service history?  Did someone top off the brake fluid in recent weeks?  What's the backstory that led you to replace the calipers and hoses?

[Rough75C20]

No brake booster on this truck. The pedal does return to the full home position after pushing down on the brake. However the pedal won’t push down very far maybe two to three inches before it is very hard to push. Just seems like the MC isnt relieving the pressure. The service history is unknown. I replaced calipers, rotors and pads because the brakes were locked up and upon removing the calipers the seals were all rotted and falling apart. Also did rear shoes, wheel cylinder and hardware. Also replaced all rubber lines as I have heard those swell over time and could cause the condition I am having.i also replaced all of the old fluid with new fluid but didn’t help. Brakes have been this way since I bought the truck a couple months ago. Was hoping the calipers were bad but doesn’t seem to be the case. I’m going to buy a new master and bench bleed and rebleed all the lines.hopefully that is the fix. The only things that weren’t replaced in the system were the master cylinder, the proportioning valve and a couple sections of metal line that were replaced more recently before I got the truck.

[bd]

I stressed that you thoroughly flush the system to remove any contaminated fluid that may cause swelling of rubber components.  For all you know someone may have added power steering fluid to the M/C before your ownership.  Sounds like you already covered your bases.  Replacing the M/C and bench bleeding before installation will likely resolve the issue.

[Rough75C20]

Thanks for the help. I’ll try replacing the master cylinder and I’ll update.

[VileZambonie]

Do yourself a favor and upgrade to power brakes.

[Rough75C20]

Is the only thing that makes the power brakes work a booster that has a vacuum line to the carb?  I believe the truck originally had a booster, but does the booster use a different master? Power brakes may be a little later down the line. Just trying to get the truck running and driving ok at this point.

[JohnnyPopper]

Ask your parts supplier if there is a differing spec between manual and power.

Yes, the only requirement for the booster is positive (full time?) vacuum from the intake manifold.

[Shifty]

The master is the same for both manual and power (assuming the 1975 C20 in your screename)