The Stovebolt Site Forums The Shop Area Driveline Stuck brakes on 48

I recently rebuilt the master cylinder and one of the rear brake cylinders on my '48 3/4 ton. Everything went well and the lines were thoroughly bled. Now, the rear drums are "stuck." When I apply the brakes, the shoes don't seem to release and they stay engaged with the drum. I've got the adjusters all the way "in" so there should be plenty of clearance. The springs connecting the shoes seem good, and everything was working fine on this daily driver until the aforementioned rebuild. Any ideas? thanks in advance.

I had this same thing happen to me on my '51 1/2 T. Turned out the master cylinder was not releasing pressure. It was pretty worn out, and had been rebuilt a number of times, so I got a new one, which fixed the problem.

Kurt

Thanks for the reply, Kurt. I put a rebuld kit in the mc. My suspicion is exactly what you mentioned. You would think the new spring and seals would eliminate this possibility. The mc was working fine before I rebuilt it, along with the wheel cylinder.

You might check/replace your rubber hoses...they can swell inside and act like a check valve, and they're cheap.

cm

Thanks, cm. I did replace the rubber hoses but the problem still remains. I'm almost certain it is a problem with the hydraulic pressure not bleeding off after the brakes are applied. Anyone have any other ideas? If not, I guess the next step would be to buy a new master cylinder.

Hy mtcb, I just found this thread, do you have a clearance between the pedal linkage and the master cylinder piston, the piston must be allowed to return to the fully released position in order to uncover the compensating port, without that port uncovered the brakes will not release. You should adjust your brakes up to where they should be, long wheel cylinder travels are harder to bleed, and they may cause the pistons to pop out of their bores, or get cocked at the end of their travel, hope that helps.

Thanks for the suggestion, Shop Shark. I'll give that a look and let you know what I find. You are right about the wheel cyl pistons. That's exactly what is happening. Maybe when I reinstalled the rebuilt mc I didn't readjust the linkage properly.

Here's an update. Pretty sure problem is in mc. After the truck sits overnight, the brakes release and wheels turn freely. After applying break pedal, breaks will not release. So, time to take apart mc I rebuilt earlier. Found several things. First: does anyone know where the very thin brass washer/seal goes. The service manual does not mention it. Second, I had the large washer on the end of the push rod installed backwards. It goes on either way and the correct way is not intuitive. Make sure you pay close attention to this. Fianally, The compensation port opening was blocked. This is literally a small pin sized hole. I think it got clogged when I honed the mc. I'll try to reassemle and bleed today and will tell you guys how it turned out.

I should have mentioned, the very thin brass washer I mentioned looks like the one that is embedded (vulcanized) in the primary seal cup. I'm thinking it maybe goes between the valve seat (a rubber gasket that sits on the end plug that screws in the mc body) and the "valve" (the thing with the holes in it that goes at the end of the spring and pushes againg the end plug.) Please help if you know where this goes.

Problem fixed! After reassembling MC (still don't know where the previously mentioned brass washer goes. If you know, please give me a shout. I took and educated guess and placed it between the valve on the end of the spring and the rubber seat on the end plug where the fluid leaves the mc.)readjusting the brakes and bleeding the lines, all is working. I used DOT 5 in the lines to avoid moisture issues. I hope that is compatible with the rubber seals in the system.

You'll be OK with DOT 5 ...

If you use a hydraulic brake light switch, tho, be prepared
to replace it every 2 or 3 years. They don't like DOT 5

In my 38 manual, it describes a V shaped ring that is between the holes on the end of the plunger and the rubber on the primary cup. This prevents the rubber on the primary cup from imbedding in the holes. This is on trucks 3/4 ton and up. Brake MC Details