Rear brakes locking up! Help.....

[legotech7]

I just got this truck, 1970 c10 short bed. the PO, had just put in new brakes all around, drums on all fours. I took it out for a 20min ride and I felt the rear rear drag a bit, so I stopped in a parking lot and noticed a bit of smoke coming from under neath, couldn't tell if it was from the rear, so I stop for a min. get back in to drive off and I couldn't move forward at all. The rear brakes locked up completely. So pondering what to do next, I sat there for about 30 min. getting ready to call a tow truck and decided to try to see if it would move, and low and behold, we were free wheeling! I drove straight home, and as I'm getting in the driveway, the same thing happened. I waited for things to cool down and took the rear drums off to see If I could see what the problem could have been. Everything on both sides are brand new no real visible wear on the shoes, but I did have to back off the adjustment on the brakes to get the drums off. They were snug but not tight! Now the PO did add a brake booster to the truck and it brakes well, maybe a little too well. Something is making them tight after depressing the brake pedal a few times but not releasing them back, they just start to lock up. Could it be the valve pictured here, might be the problem? Might I need to bleed the system and readjust the brake shoes, and back off the adjusting screw a bit? I'm a bit perplexed at this. I need the brakes to be functional and safe. Sorry for the lengthy post, but I thought a bit of detail was warranted. Thanks to all for any insight...

[mgchevyparts]

He added a booster.
Most likely the boosters front pin is slightly too long which is pushing the master ever so slightly, this will not allow the master to release. The test for this is get them to lock/drag after pressing on pedal, remove two nuts securing master to booster and pull the master forward, if the brakes now release that is the problem, A thin, flat washer placed between the booster and master (one on each stud) will usually solve this situation.

[jd7]

Originally Posted by mgchevyparts

He added a booster.
Most likely the boosters front pin is slightly too long which is pushing the master ever so slightly, this will not allow the master to release. The test for this is get them to lock/drag after pressing on pedal, remove two nuts securing master to booster and pull the master forward, if the brakes now release that is the problem, A thin, flat washer placed between the booster and master (one on each stud) will usually solve this situation.

great advice that would be the first thing I would check check

[CaptRMW]

The master cylinder looks to be the wrong one. The rod that goes into the back of the piston in the master cylinder may be adjusted too long (or the hole in the piston is too shallow) not allowing the fluid to bleed back into the reservoir. With the top off of the master cyl. you shoud see a little squirt of fluid just as the pedal is first applied. If you don't see this then the bleed hole in the master cyl. is covered by the piston being depressed without the brakes being applied. Good luck as good brakes are very important...

[legotech7]

Thank you guy's, I thought that perhaps the master cylinder booster might have been suspect, does the booster look to be too large? I haven't really looked at it to check. And what would be the right size master and booster? I will certainly try your suggestions and will post the results, I hope to resolve this promptly.... Thanks again........

[legotech7]

Well it looks like I fixed the problem of the rear brakes locking up, thanks to the forum members here. I do appreciate the expert advice. I took it for a short ride and the brakes feel better to depress. I think in the near future I will replace the MC with the correct booster, but for now I'm happy with it. Now on to the speedometer, which doesn't work and some little details to have it running safely. I have other projects I need to Finnish first. I will be posting in other threads for other issues I have with the truck. Thanks again to everyone.............

[heyjoe]

So, what was the fix?

[mgchevyparts]

Your master is good, it is a 1" bore drum/drum master.

[C5Phil]

What is fix?

[RichardJ]

>>I think in the near future I will replace the MC with the correct booster,<<

Most replacement boosters come with an instruction sheet to tell you how to check the pin length and make the adjustment. No matter what you replace it with, if you fail to check the pin you could have the same problem again.

[legotech7]

It was a simple fix as suggested by mgchevyparts in a previous post. I used a small thin washer on both side of the MC and that pushed the MC forward a bit. The booster push rod had no adjustment to it and and it kept pressure on the MC causing the braked to drag and after a while they would lock up, so the simple fix seemed to have done the trick. I plan to upgrade the brakes to disk's soon and I will replace the booster then, but for now its working OK.... As long as I on here, does anyone know how to fix, other than replacing the speedometer? Mine is not working and the PO already replaced the speedo cable, nut still no speedometer. Hope this helps someone with the same issues and thanks again........

[haysonj]

First thing is to take loose from trans and spin cable, if speedo moves problem is in trans. If it dont move then its in the speedo or cable does not engage speedo. There also is a small adapter that goes in the gear in the trans that may be gone. If all this fails then more than likely your gear is stripped.

[legotech7]

Thanks,haysonj I will check the cable. At the speedo end it feels like the tip of the cable sort of clicks in there and feels snug. I will try to engage the cable from the other side and see if it move the speedo needle. Thanks again... to be continued......