C15A brakes jamming

[harrygrey382]

Originally Posted by Bruce Parker

I bet the wheel cylinders are 'stale' and not releasing allowing the shoes to retract. Pull a wheel and a cylinder to see what's up. If the bores are clean then clean up all four wheels and the master, re-bleed and away you go. If pitted and full of crap...you'll know what to do.

Typically it goes the other way after a long sit. You apply pressure at the pedal and the rotten seals or pitting causes a pressure loss.

That's what I suspected - that's why I did what I did with the shoe adjusters. I put a lot of force on them to retract the pistons and nothing changed. So either the pistons are seized solid (I guess possible) or there's a mechanical obstruction.

Originally Posted by TCLARK

Also make sure there is a bit of "play" in the brake pedal when at the top. My CMP will sometimes not let the brake lights go out because all the pressure is not released. I backed off the rod in the brake master cylinder so there was play and problem solved.

Hmm not much play, I guess there's a tiny amount but the pedal is right at the top

Originally Posted by Richard Farrant

This can be proven easily by releasing a bleed nipple on a wheel and seeing if it will then turn, if it does not then it is a seized wheel cylinder.

Yep that's what I tried first up, no change. As above I guess you're right it could be a (well 4) very very seized cylinders.

Was liking the idea of the master cylinder return washer in the wrong way round, if releasing the pressure made a difference I'd be right on it. Now I just have to go and pull a drum off... Is going to be fun with these tight shoes

[Lynn Eades]

Probably the (aluminum??) pistons went in dry, and have grown (oxidized)in their bores.

[harrygrey382]

Originally Posted by Lynn Eades

Probably the (aluminum??) pistons went in dry, and have grown (oxidized)

They went in with rubber grease, could they still have oxidised?

[rob love]

12 years on a rebuilt hydraulic system is a long time. Agree with the others who say to try and crack a bleeder screw and see if the pressure leaves, but most likely you need to take er apart and rebuild it again.

Not sure about your neck of the woods, but hydraulic components are dirt cheap these days. Wheel cylinders run around $30-35 each. A Ford master cylinder is about the same price. Unfortunately not the same deal for the chev master cylinders at this point in time.

[chris vickery]

I had a similar problem on my first 12 cab when I had just fully rebuilt the braking system.
My pedal had little free play and the brakes worked great for a little while. After a couple miles of driving the brakes would stick on and the drums were heating up.
Investigation reveal led improper adjustment of the pushrod which was not allowing the fluid to release freely back into the master cylinder. As the fluid got hotter and continued to expand the result was application of the brakes by sheer hydraulic pressure buildup.
Once the pushrod was adjusted the brakes worked perfectly.

[rob love]

Adjustment of the master is critical, as has been mentioned. Another possibility is the collapsing of a flex line internally. They will allow fluid to go to the cylinder, but not back again. If it is the line to the rear axle assy, then both will lock on. To find this out one has to raise the vehicle and determine which wheels are dragging. But again, on a 12 year old rebuilt system that did not get a lot of use, the cylinders would be my prime suspect.

[Phil Waterman]

Hi Harry

Assume that your problem is with the rear wheels?

First my HUP has almost always had a rear brake grab when the brakes are first applied after the truck has been sitting for any length of time. Now if I remember once I pull the truck out of the garage I stab the brakes a couple of times and the problem goes away, suspected cause light rust on brake drums from sitting that cleans right off.

Your problems as other have suggested sounds like miss-adjustment, cylinder sticking in the bore, failure of the brake pressure to release, or two other causes not mention yet.

Weak or wrong length brake springs
Parking Brake, is your parking brake on the drive shaft or on the wheels?

As other have suggested very careful brake adjustment by the book is probably the best approach. Suggest that you block the truck up so all the wheels are free to turn then back all of the adjustments off both top and bottom adjusters and start fresh at each wheel.

You comment or question about something simple like brake linings being to thick would indicate to that the linings are new or really good condition, correct?

One last point, many years ago I had all my cylinders wheel and master sleeved this has ended problems with the cylinders. Now it is replace the rubber parts every 10 years on principle.

Keep us posted on what you find.

Cheers Phil