Well guys I posted this awhile back and I changed the prop valve that was suspect but I'm still having the same problem?

I have put in two used prop valves from the boneyard, new rear wheel cylinders and shoes, front pads, and new master.

Yesterday I thought I fixed the problem and the brakes seemed better not overly sensitive like before. They where sensitive but seemed to be just nice and firm. Today in the rain I can tell that I have the same problem. Too sensitive and the backs lock up easily.

The wierd thing is at least with the old prop valve is that it seems to be worse after being parked for awhile, then it gets somewhat better after driving. Too early to see if the new one will have the same behavior???

One other area that might cause problems is that I have the optional 13 inch brakes and they use a different prop valve then the standard brakes. I'm not sure if the donar had 13inchers or not???? I might still have the wrong prop valve???

Anyone got any clues on where to look next???? Maybe the master or booster???? I already checked with GM on the prop valve and I might not be able to get one.

Thanks!!

I would check to see if the rear drums are out of round unless you have replaced them.

I had the drums turned when I did the rear brakes? Do you think it would be both sides?? I hate taking drums of a 3/4 ton!

Are you sure everything is clean inside the drums? I had an 86 1-ton that did the exact same thing, sensitive rear brakes and early lockups. Especially in wet weather. Turned out to be some grease and grime inside the drums from the axle tubes. Got it all cleaned up and rebuilt, and no more problems! Thankfully! I was tired of the backend sliding around in the snow! (while braking.... smile.gif )

Bulb122,

I don't know dude! I hate to pull those drums again. Originally that was one of my problems but it never locked up like that even with the gear lube soaked shoes that it had prior to the brake job. In fact it drove fine that way! I put new bearings/seals cleaned the crap out of it before assembly.


I'll check the backing plates today at lunch for signs of fluid. I might pull drums this weekend, at least process of elimination??

Thanks!

My 81 1 ton dually does the same exact thing. I am in the process of putting all new tubing in the front ( few small leaks) while replacing the master.

Ponjon,

looks like where in the same boat! Are you hoping the master will fix your prob?

We had that problem when we used Bendix shoes on some trucks here at work, switched to NAPA shoes and no more problems.

1974 was the 2nd year of that particular generation of truck, but in the braking department a new thing made it's debut for 1974: Hydro-Boost system. For higher GVW models (Camper Specials and such) Hydro-Boost was a new system where power assist was derived by hydraulic pressure from the power steering pump. Various brake components were matched by GVW, like larger calipers & rotors, tandem instead of single vacuum booster, larger finned rear drum brakes, and hydraulic instead of vacuum boosting. I'm wondering aloud if that might affect you in your situation when selecting brake components.
1974 also marked the debut of the new disc brake wear sensors

Good info Cam, is this a great site or what. graemlins/thumbsup.gif

Thanks for the info Cam!! I have seen that system at the bone yards. Maybee some of the back brake hardware could be similiar but the master and booster is totally different.

But hey I'm willing to follow any and all leads!! the first thing I'm doing this weekend is to make sure I have the right prop valve, then either new master or pull drums and look for leaks and different shoes?????????

Hilljack