brakes dragging question

[RustyBucket]

A pretty serious illness left my 2 old trucks pretty much abandoned in the shop for a year+. Recently I charged the batteries, primed the carbs. washed 'em off and amazingly both still run/shift/drive/ pretty good. Aftermarket A/C on the '71 GMC still blows cold. Driving the GMC the other day & stopped and talked to a neighbor for 15min or so. I left the truck running w/the air on and the temp. got up to about half way on the factory gauge. I started to back out of his driveway and the truck didn't want to roll, the front brakes were dragging. I barely was able to drive around to my driveway 'cause they were almost locked up. I left it running, raised the hood and ran cool water all over the radiator for awhile. I brought the temp. down to the first mark on the gauge, closed the hood and the brakes were fine. Yesterday I backed it out & let it sit and warm up to almost halfway on the gauge. The brakes bound up again. Truck has factory pwr. disc brakes. I'm not healthy enough to work on it much yet, but has anybody had similar brake trouble? THANKS in advance!

[Caddylackn]

Brakes dragging could be two things? Rusty caliper bores causing the caliper piston(s) to stick, or bad hydraulic brake hoses. The hoses bulge and hold residual pressure causing dragging. Could be a a combination of both.

Both of these are easy to fix, new calipers, and new brake hoses. If you buy new calipers, you should definitely replace the hoses as well, since you are right there and need to bleed the brakes anyway. Any original brake hoses should be replaced at this point. There is one above the back axle.

Now is the time to flush and replace all brake fluid, as water absorbed in the brake fluid is how the caliper bores got rusted in the first place.

[Steeveedee]

If it's just the temperature difference that makes them lock up, I'd suspect moisture in the brake fluid.

In '71 the pistons in the calipers are stainless, so they shouldn't be rusted. On GP, I'd start with the hoses (all three) if they are over 10 years old. Stuff just isn't made like it used to be. That'll require fresh fluid. It won't pinpoint the actual problem, though. If you're concerned with root cause, just start by flushing out the brake fluid with fresh. You won't lose that much fluid if you then replace the hoses.

[RustyBucket]

Originally Posted by Steeveedee

If it's just the temperature difference that makes them lock up, I'd suspect moisture in the brake fluid.

THANKS! It DOES seem to be temperature related. I've had the truck 10yrs or so and have put brake pads & shoes .. & packed bearings on it a long time ago. No leaking calipers or wheel cylinders that I recall. I guess I've never flushed the system. I drove it in a Christmas parade a few years ago and creeping along raised the engine temp. and the brakes started dragging.... let it sit till it cooled off & it was fine.
My next project will be the '72 Blazer's brakes. Washed it off & drove it yesterday again, first time in a year+. It ran & shifted ok but feels like maybe no frt. brakes. I feel very lucky that the gas didn't go bad & make more work for me.

[LNP]

As long as the system has never been left open for any length of time there shouldnt be any moisture in the fluid. Brake systems are sealed air tight so they dont leak and contaminates can not get in. Aged hoses, broke down brake fluid, frozen cailper pins, or caliper pistons stuck to the seal from none use would most likely be one or more of the issue. If you do feel the brake fluid is contaminated then you will need to for sure disaasemble the whole system flush out with brake cleaner and replace everything rubber. That would be all hoses and seals including whats in the master cylinder. Contamination can cause seals to fail, swell, leak etc. Better safe than sorry when it comes to the brake system.

[LockDoc]

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Heat source too close to the brake line. (ie: exhaust manifold, exhaust pipe) Expanding the brake fluid in the line. The next time it does it open one of the front brake bleeders and see if it releases the pressure on the calipers. Easy enough test to see if that may be the cause.

LockDoc
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[LNP]

Originally Posted by LockDoc

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Heat source too close to the brake line. (ie: exhaust manifold, exhaust pipe) Expanding the brake fluid in the line. The next time it does it open one of the front brake bleeders and see if it releases the pressure on the calipers. Easy enough test to see if that may be the cause.

LockDoc
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Good point never thought of that one. Maybe if it has a exhaust leak as well and the leak right on or at a brake line or hose.

[Reddeye]

I had this issue as well. After chasing the issue upstream gradually replacing calipers, flex lines, and hard lines, the issue was finally solved by replacing the master cylinder.

[RustyBucket]

I've just started driving again after being sick. I'm really thankful that I've regained enough strength & balance to mess with my old trucks some. It had been a long time & I'd forgotten how much fun the Blazer is. Hopefully I'll continue to improve & the weather will cool off some. Thanks for the responses.

[72SB]

Regardless of the cause...service of brake system sounds old....if much at all and the truck sat for a year...

Replacing soft lines, rebuilding or replacing calipers and at that point replace pads and turn or replace rotors if needed. Don't forget rear wheel cylinder rebuild if needed also

Brake fluid absorbs moisture and does need to be flushed every 3 years ideally, milage not the factor, its time. Brake fluid should be amber in color...not black.

A complete brake flush "may" resolve the issue alone

IDK what heat espouser issue just "started" to cause this but check to confirm a soft line isn't near, touching, exhaust

[Oregon 1967 GMC]

**** happens when they sit for that long. Good answers here.

[Accelo]

Check for a slight miss-adjustment on the rod between the pedal and the master cylinder can cause brake lockup. Or, if you can, back off the adjustment a turn or so.
It doesn't open the return port like it should if this is out of adjustment.

[LockDoc]

Originally Posted by Accelo

If you can, back off the adjustment a turn or so.
It doesn't open the return port like it should if this is out of adjustment.

I probably should have added this in my post. That is what actually causes the external heat to build up pressure in the lines.

LockDoc

[RustyBucket]

Originally Posted by Accelo

Check for a slight miss-adjustment on the rod between the pedal and the master cylinder can cause brake lockup. Or, if you can, back off the adjustment a turn or so.
It doesn't open the return port like it should if this is out of adjustment.

I read this response and did a site search about how to adjust... and came across this 2019 post by AussieinNC...
"1/ Suggest you unbolt the master cylinder from the booster and place one 1/16" flat washer on each stud, then replace master cylinder. This will space the master a little further away from the booster..."
Thought I might try this because I can stand up and do it. I'm not up to working under the dash yet. The brakes function fine normally. I only have this problem when I let it sit & idle.

[Minkota]

Originally Posted by LockDoc

I probably should have added this in my post. That is what actually causes the external heat to build up pressure in the lines.

LockDoc

I had this issue on my 67. the brake lines going down from the Master went down on the inboard side of the frame and was almost touching the header.