Brakes lock up only when it rains...

[dabruce15]

It's a 72 Shortbed Stepside, front disc, 250 six, 3 speed. I added power brakes (CPP booster and M/C) and now it locks up with the slightest touch when it's raining. Drives and stops fine in all other conditions.

Any ideas why this is happening? Any help would be greatly appreciated.

[dabruce15]

Here's a couple pics of before and after the swap...

[custom1970]

What's locking up front or back? And have you ever replaced all rubber brake lines? also, see hot your brakes get next time your driving around, i bet they're dragging a little bit all the time

[dabruce15]

Originally Posted by custom1970

What's locking up front or back? And have you ever replaced all rubber brake lines? also, see hot your brakes get next time your driving around, i bet they're dragging a little bit all the time

I'm pretty sure it's the rear.

[custom1970]

if you haven't yet, replace the rubber brake line that goes around the rear end. They have a tendency to swell shut over time and act like a check-value

[dabruce15]

Originally Posted by custom1970

if you haven't yet, replace the rubber brake line that goes around the rear end. They have a tendency to swell shut over time and act like a check-value

I haven't replaced any of the rubber lines. I'll give it a shot. Thanks for the info. What elso could be causing them to drag?

[ChevLoRay]

First thing...disc brakes will always have a slight "drag", in that it's just what they do. However, it won't be a drag in the same sense as a shoe-type brake that doesn't release completely.

As to what it is that can make a brake drag: As suggested, a flexible (rubber) hose that has collapsed internally will restrict the out-flow of the brake fluid as the brake shoes (or disc brake pads) retract after use.

Anything that may have leaked onto the shoes. If you have a grease seal in the rear that is leaking, or if your wheel cylinder is leaking slightly, then anything that accumulates on the lining can make it grab.

Weak hardware (springs, etc.) may not allow the shoes to retract completely. If so, and the shoe(s) drag, they could be premature in their braking action.

Drums that have been cut (turned) unequally from one side in comparison to the other. If the diameters are not identical, or nearly so, then one side will brake more than the other. Trust me on this one....been there. Not pretty.

Your proportioning valve may not be doing its' job. After all, it's at least 39 years old.

Wait for some dry weather, then take it out somewhere that doesn't have any vehicular traffic, like an abandoned mall. You get the idea. Then, start locking up your brakes to see which one it actually is, if you don't already know. Once you know for sure which wheel is locking, then you'll know where to start.

[cdowns]

pump the brakes instead of just slammin them to the floor you stoping distance will decrease

[y5mgisi]

the metalic brake pads or the front discs are flash rusting in the rain and greatly increasing the coefficient of friction

What he said. All three of my trucks do this. Its something to do with the moisture and making the brake shoes/drums rust just a little tiny bit. Enough to add in a little extra drag. after the first couple few times i hit the brakes, they are like normal.

[Papaloo]

My 2003 GMC Sonoma does this. I'm pretty sure either the metalic brake pads or the front discs are flash rusting in the rain and greatly increasing the coefficient of friction. At speeds below about 15, if you lightly touch the brakes, it stops the truck dead in its tracks. I'll normally lightly apply the brakes with my left foot while the truck is under power to clean and dry them. Once I do that they're fine.

[stich626]

I only see one brake line coming from your new master???

[dabruce15]

Originally Posted by stich626

I only see one brake line coming from your new master???

There is two. The one you can see in that picture is the one I replaced that I broke when removing the old one. The other one can barely be seen because it's covered in "age".