Rear brake lock up??

[kenm78]

Never seen this before. I have a 71 C10 LWB originally came with manual brakes I got a power brake set up from a 72 C20 LWB. Was working great for the last two weeks, then all of the sudden I started breaking today and my rear axle locked up when I braked I let off and it released?? At first I thought my proportional valve went bad. I popped the master cylinder cap and fluid poured out the front chamber in the master cylinder. It was overfull and the rear one was about 1/2 low. I took a little fluid out of the front part and put the cap back on and its not locking up any more. Any one every have this happen or experience with this?? Know of any possible broblems or is this common?

[kenm78]

???

[brianthelion02]

I would be for pullin the rear drums and see if there is any fluid on the linings!!!!!!!!!

[smoked68]

I have the same problem, checked every thing I can think of and some of the ideas from the post. Fluid is full, no leaks but rears will lock before the fronts if you hit them hard. Oh yeah, the idiot light on the dash comes and goes. Right now its on, may be off when I leave work today??? I am very interested in the responces here. Mike....

[cdowns]

bad master cylinder

[ksjh1316]

Have the same thing going on, on my GMC pass rear will lock up cant find a darn thing wrong with the whole system. Hopefully yours is easier to find than mine dang tire is starting to get bald fast

[Wreckless_71]

1316... is it locking up all the time or just sometimes? Mine will lock up everyonce in a while but only when it's really wet or cold. I talked to a guy about it and he said the drums get a little thin layer of rust on them and they grab for a little bit, usually a good healthy one tire fire helps!

[chev74]

my suspicion would be a leaky brake cylinder in a drum. mine does it too. every once in a while when i start to drive the first few times i even TAP the breaks it will lock up. once it warms up it works fine.

[smoked68]

Hi guys, mine does it all the time, rears grab before the fronts, wet roads are much worse than dry, I have replace everything, master cyl, wheel cyl. checked the lines, and no leaky leak I can find. This is frustrating. But it is either worse @ start up or my foot is just heavier then?? Mike.........

[chris71]

same here, pass. rear locks up when you hit the brakes hard. i know that i had this problem before and when i converted to power setup it felt like it was worse, i think the ease of braking with the power setup "amplified" it some. i always assumed it to be the wheel cylinder locking up (due to age), this weekend i am going to put in new wheel cylinders and make sure that the adjustment is correct for both sides. i put in a brand new master cylinder a month ago because the old one was bad and you could tell (the pedal was real spongy), no luck, pass rear still locks up. there is a blurb in the chasis manual that explains the master cylinder front/rear failure cenario. i will check this out and keep you guys posted on my outcome. i drive mine daily so there isnt a chance for rust to build up on the drum surfaces

smoked 68: when that light comes on that means that there is a pressure drop in the fluid (bad), and since the MC creates the pressure, i would guess that your MC is bad or going bad. a MC costs $40, i would just replace it IMHO

[rlm316]

Were the original brakes on your truck drum/drum and was the donor truck drum/drum also? Did you change the front brakes to disc and is the prop/dist valve original? Drum brakes work off of a residual pressure and disc do not. Is it both rear wheels locking or just one? With the fluid levels in the MC changing places you should look for a failure in the areas where there is a common location. It sounds like your MC has failed and is letting the fluid past the piston and into the other resivoir. Also check the self adjusters in the drum brake wheels. They will gum up/ rust and not operate properly. I take mine apart every time I change shoes and clean /lube them with never-seize.Also check the star wheel on them for wear and replace them if necessary. Your rubber flex lines can swell on the inside and act like a check valve letting pressure in but not out. Leaky wheel cylinders will make the affected brake slip after the shoes/ drum are soaked with fluid. Look for wet spots on the backing plate opposite of the problem wheel. Good luck and I hope this helps, Lane

[Winter]

Several different issues of rear brake lock up are being mentioned. Where the rear brakes generally lock up before the front, especially in wet weather may be a front-to-rear brake proportioning concern. Anti-lock brakes or an adjustable proportioning valve will help resolve this issues. The ideal proportion, with the fronts locking up just before the rears, changes with wet/dry conditions and the payload carried by the truck.

My 1969 GMC step side had another issue, the rear brakes remaining locked. After rebuilding the brakes, new shoes and wheel cylinders, I discovered the actual problem was sticking parking brake cables. All three cables, the one leading from the cabin, and the two wheel cables were almost completely frozen in their sheaths. When the truck was backed up in reverse and the brakes applied, the automatic brake adjusters would tighten the brakes and the frozen hand brake cables keep excessively tight tension on the brakes, causing them to remain locked. I worked free & lubricated the cabin cable and replaced the two wheel cables.

[cdowns]

all great theorys about the brakes not one has addressed why the rear compt is overfilling into the front of the master// bad master

[smoked68]

I have no idea as to why the rear is filling to the front unless there is some type of back pressure in it to push the fluid to the front. On the other note I just replaced the MC and every thing else (with exception of the lines), MC, whl. cyls. new calipers. I just dunno. Lite comes and goes, fluid is always at correct level...???? this has been going on for months, stops great other than the rears locking and that bloody lite, electical tape maybe......hum.

[chris71]

Originally Posted by smoked68

I have no idea as to why the rear is filling to the front unless there is some type of back pressure in it to push the fluid to the front. On the other note I just replaced the MC and every thing else (with exception of the lines), MC, whl. cyls. new calipers. I just dunno. Lite comes and goes, fluid is always at correct level...???? this has been going on for months, stops great other than the rears locking and that bloody lite, electical tape maybe......hum.

woops read it too fast, ok, then my guess for your problem would be a faulty prop valve, have you replaced that too? it could be saying there is a pressure issue when in fact there isnt. dont forget, just because we have new parts, doesnt mean they work right like they are supposed to

[QUOTE=kenm78] I popped the master cylinder cap and fluid poured out the front chamber in the master cylinder. It was overfull and the rear one was about 1/2 low. I took a little fluid out of the front part and put the cap back on and its not locking up any more. Any one every have this happen or experience with this?? Know of any possible broblems or is this common? ]

Ken,this is not common, fluid level itself doesnt have any affect on the brakes locking up, lack of fluid= you will have no brakes. it may be a piston/seal issue inside the MC such as rlm316 has suggested, that causes this fluid change, if the MC is used, like mine was, rebuild or replace IMHO.

[smoked68]

I have not changed the prop valve as yet, I have another one and guess that is the next move. Thank you one and all.....Mike.......

[Myself]

My wife's '68 with disc/drum does this on the rear right too. Usually if it's been really foggy or raining. Also my '88 S10 Blazer does it, and my buddies '95 Tahoe, and another friends '94 Chevy truck. It seems this is a common problem on trucks and SUV's. I believe it has to do with the prop. valve set up. No matter what you do if it is the right rear locking up the only way to fix it is to adjust the shoes on that side a little loose until it quits. I have also used channel locks to crimp down that line a little at a time 'til it quits. That is assuming everything else is working fine or been replaced. My wife's '68 has everything brand new to the braking system front to rear and still does it from time to time!!

[Old Chevy Mech]

Crimping the line? That seems a bit risky to me.

Frank

[68gmsee]

Not sure if this has any bearing on the problem, but many years ago, I joined this forum to address a similar issue that I had. After checking all of the recommendations, I found it to be a bad rear brake rubber hose. The one on top of the rear axle.

The inside of it was swollen so bad that brake fluid under pressure would pass through but could not get back to the reservoir.

[Myself]

So does riding around in a truck with one wheel locking up. lol.

[smoked68]

You know now that you mention it I really dont remember if I changed that hose?? I shall investigate asap...thanks for the idea, that might be the idiot lite issue also............

[ChevLoRay]

So, I wonder how many of us are driving around in our trucks while we relying on 35+year old parts to keep us safe. How many of us have done a complete brake job on the rears (turned/replaced drums, new shoes, hardware, etc) then hooked it all back up to those old flex brake lines that GM put on it when it was on the assembly line? Geeze, we take a lot for granted.

My truck was also locking on the right rear in a near-panic stop before converting the front to discs. Since then, the fronts will lock, first.

[kenm78]

Thanks everyone good info I will do a good inspection first and see what all needs to be replaced.