More brake problems 89 Jimmy

[2Fst4U]

I have an 89 fullsize GMC Jimmy with front disc, rear drum brakes with the proportioning valve mounted on the cross member in front of the engine. I noticed about three weeks ago when wheeling, the rear brakes went out. I reversed a couple of times to try and adjust the rear drums. This and waiting about 10 minutes seemed to resolve the issue. I tried to drive around my neighborhood last week in a snow storm and again, no rear brakes. I put the vehicle on jack stands and when applying the brakes, the rear wheels barely come to a stop. I have bleed all four lines and did get sufficient fluid from the rear brakes. The proportioning valve button did not appear to be tripped. I also tested it according to the manual by grounding out the lead wire which triggers the brake light on the dash. The light was not previously on. I did notice in the process of diagnoising this, with the vehicle on jack stands, if I would "stab" the brake pedal, the brake light would flicker on for a split second. After bleedign the lines, the pedal felt a little soft. I'm pretty confident I did not introduce air in the system, but it could be a possibility. I also adjusted the rear drums to see if that was the issue, no change. I have read all threads about the proportioning valve and master cylinder issues and have a master cylinder on order. Has anyone else had these same symptons? What was the fix? Also, I have checked the power booster and it appears to be ok. I'm hoping its the master cylinder, but not sure. If its the proportioning valve, does anyone know where I can get a replacement. I have checked every parts store known to me and the dealership. It has been discontinued. I thought about going to an adjustable one as one day I hope to do an axle swap and rear disc conversion. Any help/guidance would be much appreciated.

Your's Frustrated,
James in Parker Colorado

 

[4X4HIGH]

Sounds to me like you need to adjust the rear shoes up correctly. If the shoes are out of adjustment then there isn't enough movement in the wheel cylinder to make the brakes grab. Also the self adjusting brake mechanism RARELY works correctly so do not rely on that to adjust the shoes up for you in reverse.

 

[2Fst4U]

I adjusted them by hand as well while it was on jack stands. I adjusted them to the point where I could not spin the drum by hand. This had little to no effect on the problem. The brake shoes also have a good amount of life left on them. Let me know what else you think. I appreciate the feed back : )

 

[4X4HIGH]

If you've adjusted them by hand like you said and you can't get sufficient pressure then it has to be a bad master cylinder if you get good fluid flow when bleeding the wheel cylinders.

The master cylinder is what produces the pressure, the proportioning valve distributes the pressure at a given perctange, and the wheel cylinder moves the brakes because of the fluid flow/pressure.

A master cylinder can bleed back within itself while not having any external leaks or drawing any air into the system.

 

[2Fst4U]

I'll let you know how it turns out. I should have the part by this friday. Thanks for taking the time to reply.