Goofy brake action on my Chevy pickup

[Rich69shortfleet]

My 69 C10 has 71 stock front disc brakes. Last Summer I had some serious brake issues and ended up replacing the rear shoes, the rear cylinders, the master cylinder and the front pads. I also had a lot of crud built up in the rear lines on the axle so I removed them and cleaned them out. I also cleaned out the proportioning valve as there was some small amount of rust in the fluid coming out of the old master. About the only thing I did not replace were the front calipers as they seemed in good condition. The proportioning valve is not available as a reproduction or normal part replacement. By the time I got done bleeding the system, it most certainly has fresh fluid throughout.

Just recently the brake warning light on the dash has been coming on occasionally and when it is on the brakes feel soft and don't stop as firmly as they should. If I press the pedal hard 2-3 times the light goes out and normal braking is restored. For a while. The booster is good, this is not an issue of a lack of power or a hard pedal. When pumped up the pedal remains firm and does not sink to the floor under constant firm foot pressure. I did 2-3 three hard stops in reverse to adjust the rear drums but this didn't help.

There are no fluid leaks at the cylinders or calipers or at the valve or master. The fluid levels in the master are constant.

Could this issue be related to the front calipers, the only non-new part in the system besides the prop valve (which I can't replace anyway)? Could a sticky caliper cause this? I just don't know what else to try. Maybe the master, new or not, is bad? When working correctly the pedal is high and firm and the truck stops very straight and very well.

[Rich69shortfleet]

Funny thing, someone suggested in a Youtube video to mash the pedal and while keeping it mashed whack on the prop valve with a pair of hammers. I figured this couldn't hurt so I tried it. The truck then sat for an hour and when I checked it just now the pedal is high and firm and no light. It usually only needs to sit for about 10 minutes before the pedal went soft again. We'll see over the next few days if this "repair" sticks. In the mean time I'm going to dig around for my receipt for the master.

[kwmech]

Rear shoe adjustment? How does the parking brake work?

[Rich69shortfleet]

Parking brake works fine. I use it often as my driveway has a moderate incline.

[old Chevy guy]

I have seen more than one master that was new that was defective. Especially if "new" is remanafactured. There was a thread not to long ago someone was bending their brain trying to figure a problem and it was just that,faulty "new" master. If nothing makes sense sometimes it's the obvious even when that doesn't make sense. Just my two cents.

[ak808]

Subscribed!

I have the same problem, and for the life of me cannot figure it out. Pedal will be nice and firm and have excellent braking, next day on the drive to work it wont stop itself at an idle in gear.

I don't get how the problem can come and go!

[Rich69shortfleet]

When I first did the disc brake conversion I went through three masters before I got one that worked well. This one worked well once bled. Given my hammer treatment to the prop valve I'm wondering if it is in the prop valve. Maybe another removal and cleaning is in order.

After it sat for a few more hours last night I tested it again. Pedal high and firm just like it should be and no light on the dash.

Why doesn't anyone reproduce these prop valves? Even a rebuild kit for them would be helpful.

[Rich69shortfleet]

Master I installed last August was new. Anyway, requested a warranty replacement. Hopefully that will resolve the issue. When I tear it down I plan to also remove the prop valve and clean it out as best I can. If this doesn't resolve it, I don't know what else it could be aside from the calipers and I don't see how they could cause this kind of problem.