86' Brake drums lose pressure - Fronts don't

[HDL_CinC_Dragon]

Hey guys, I'm having a weird issue. When I press my brake pedal to maximum, my rear drums activate normally but only for literally half of a second before releasing despite still having the brake pedal down. My front brakes do not lose pressure at all and maintain their hold. I do not believe this is a new problem either. I've been having weak brake issues for a while now and finally this weekend replaced them entirely on both rear wheels. The replacement went smoothly and I made sure to properly bleed the lines and I have visually confirmed there are no fluid leaks anywhere in the entire bumper to bumper brake system to include inside the drums at the wheel cylinders.
It's not my master cylinder because if it was the front discs would lose pressure too. I dont think it could be a clogged line because if it was then they would simply just not get the pressure in the first place. This confuses the heck out of me! Anyone got any ideas?

 

[77'911]

You might consider checking the bias valve. You might consider replacing the hoses and flushing out the system. I replaced the hoses a while back on our 1990, and it really helped the pedal feel. It's also worth checking the Booster and the vacuum off it. Best of luck with it.
regards

 

[KidVermicious]

Your LSPV could cause that - it's designed to monitor the distance between your rear axle and the bed of the truck, and bias power to the rear brakes the lower the bed is. If it's loose or otherwise malfunctioning it could be causing your trouble.

How's the pedal feel? Does it get squishy? The front brakes are inherently better than the rear, so if the booster is starting to lose vacuum it could feel as though the rears are failing while the fronts don't?

Don't rule out air or debris in the line, either. Bleed them again. And again.

 

[nevada]

the master cylinder is bad.

 

[HDL_CinC_Dragon]

Thanks for the replies! Is the LSPV that random little brake line that runs from the rear line manifold above the passenger tire all the way across my axle and terminates completely randomly into some booted circle thats bolted to a movable armature on the other side of my differential? Hahah I was reeeeelly wondering what the hell that thing was.

The pedal does seem to get a little squishy when I pump the brakes. My engine struggles a lot too when I pump them a lot but I'm guessing thats just because the booster is sucking the life out it.

Sometime this year I plan on redoing the entire brake line system with shiny new lines (Most likely steel braided). I did bleed them quite a bit but perhaps I'll do it again anyway!

I realized upon looking at the master cylinder that the front brakes and rear brakes have their own lines coming out of it. I thought one was a return line and that both the front and rears were receiving pressure from the same single line so I bought a new master cylinder yesterday afternoon but I won't get a chance to swap them out until later on today. I'm hoping that solves my issue. I'll report back this afternoon when I've done that (I have like 2 gallons of DOT3 too just in case )

 

[HDL_CinC_Dragon]

Hey guys, sorry for the horrendously late update. I swapped out the new Master Cylinder after taking apart the old one revealed the seals on the pistons inside were completely shot. I could have just gotten new seals but a whole new assy was just $50 so I figured why not. Bled my brakes after the install and it was good as new! Thanks for the help guys!