What's making my front brakes lock up and stay locked up?

[AirSpeed]

Driving down a curvy mountain road the other day and applied the brakes to slow around a corner and my front brakes wouldn't release. I pulled over and tried going back and fourth hitting the brakes and they finally let go. This is on my 72 GMC K1500. I have power disc brakes on the front and just replaced the calipers a couple years ago. I looked for anything lodged in them but found nothing. This has since happened a couple more times. Any idea what would cause this?
Posted via Mobile Device

[Southcity]

I would start with a good visual check of all your brake components:

1. Is the fluid clean and at the correct level in the master cylinder?
2. Do you see any leaks/wet spots around the master cylinder, proportioning valve etc.
3. Follow all the lines from there down to the front calipers, any leaks?
4. What is the state of the rubber flex lines between caliper and hard lines? are they old and cracking or are they newer and replaced when you did front caliper replacement? are they binding up anywhere?
5. Break a bleeder screw loose on one of the front calipers, does fluid come out? maybe have a friend pump the brakes when you break a bleeder screw loose to see if you get fluid.

Eliminating some of the above should help you know where to concentrate the troubleshooting efforts. Hope this is helpful, let us know your progress as others will chime in to help out.

[too much stuff]

I would start with the rubber hoses. They can collapse inside and hold pressure if they get bad...

[AirSpeed]

Originally Posted by too much stuff

I would start with the rubber hoses. They can collapse inside and hold pressure if they get bad...

The hoses don't look bad on the outside but I'll check them out. Unfortunately all my tools are two hours away until Monday, took them to my daughters house to put a new engine in her 69 c10. I've checked the fluids, master is full, no leaks, no kinks in any of the lines, fluid looks clean. I replaced the master and booster a couple of years ago and they seem to function properly.
Posted via Mobile Device

[custom10nut]

Hoses and cylinders
both are cheap and easy to replace.

[raggedjim]

I also vote hoses, been there done that...

[RGT]

I have had hoses that look good on the outside do it also, the inside lining separates and it acts like a check valve.

[Aptosdude]

Sticking pistons? Maybe the calipers need rebuilding.

[towbar]

Here's a quick and dirty way to tell if its the hoses or calipers. With the brakes stuck, open the bleeder valve on the caliper....if it releases / frees up, then its the hose. If it doesn't, then its the caliper.

[midniteblues]

I've had the hoses lock up the brakes like a one way valve.

And I've had a sticky piston in a master cly do it too.
This can get tricky during bleeding.

[HO455]

Were both brakes stuck? What were the symptoms? The odds of both hoses or calipers being bad are pretty high. Even if it was a bad hose on one side and a bad caliper on the other side. I would look at the master cylinder first. Or a pinched line between the master cylinder and the Tee on the front cross member would be easier to rule out. It is also possible that you have a drum brake master cylinder and it has a 10 lb residual valve that is trapping fluid and as the fluid gets hotter it expands and forces the caliper to apply the brakes. Once the fluid cools the pressure fades. This kind of problem only occurs when you are really working the brakes. Hope this helps.
Good luck.

[davischevy]

Hoses.

Been there and done that.

Several times.

[LockDoc]

-
I just had that happen on my '72 Dually. Mine was only the right front but I replaced both hoses anyway. Hoses looked good, but I haven't had any problems since.....

LockDoc

[special-K]

One hose can start collapsing before the other but you don't notice till the other does. Lots of originals still on these trucks that look great. It's not unusually for both to have the issue after 45 years.

Opening the bleeder(s) test will tell the tale.

[67 chevelle]

Originally Posted by towbar

Here's a quick and dirty way to tell if its the hoses or calipers. With the brakes stuck, open the bleeder valve on the caliper....if it releases / frees up, then its the hose. If it doesn't, then its the caliper.

It could be many other things besides hoses .


If you replaced the master and its too close to the booster it will do this same thing , usealy when hot , if you have rear drums , drums are kind of self actuating , and your hose from the dif to the crosmember is bad this could very well happen , they will lock or just not work at all . I assume the front brakes are newer , so those hoses are newer .

[snipescastle2]

Sounds like collapsed hoses to me too.
Ben

[AirSpeed]

Thanks all! I'm going to replace the hoses, shoes and calipers. No sense messing around! Found rebuilt calipers at NAPA for $25 each, probably cost $150 to do everything so I may as well get it right the first time. Master cylinder is new as is the booster so hopefully I won't have to mess with those!

[AirSpeed]

Turned out the caliper was seized up. The piston was absolutely stuck. I took the old hose off and it still wouldn't move. I flushed all the old fluid out of the system to make sure no crap was in there. I went ahead and replaced the callipers, pads and hoses. Need to bleed them again as they're a little spongy but my helper (wife) had to leave. Hopefuly I can get the proportion valve centered, the light is still on.

[Stocker]

To center that prop. valve try stomping on the brakes really hard. Often does the trick.

[davischevy]

I'm sure you will get the brakes working properly. But to clarify, is the wife coming back?

[Aus69c20]

This is unlikely.
But I had the same issue and it ended up being a split pin behind the booster rotating and jamming against the firewall. I put in a smaller pin, folded it over good and no more problems.

[davepl]

I dunno, he says "front brakes", as in both, which can't be the hose (unless both magically did the same thing at the same time, highly unlikely).

If it's both, it's gotta be in the master, doesn't it?

[Aus69c20]

Yeah I agree davepl.
But I would have said it was my front brakes as well ( they were smoking).
I figured though unlikely it was the easiest to check.

[lolife99]

Originally Posted by too much stuff

i would start with the rubber hoses. They can collapse inside and hold pressure if they get bad...

x2

[davischevy]

The son in law had a caliper lockup on his Blazer a while back. He changed the hose and fixed it.

I says "why didn't you change both hoses", he says "nothing was wrong with the other one".

The next week the other hose went bad.