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Rear Brakes

V

Voless

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Still fighting the brakes on this 78. Went through the rear drums completely, fixed all leaks in the lines. Flushed and bled the brakes a few times. Put a full bottle and then some through the system. But the rears don't seem to be working. With the truck jacked up and pedal pressed the wheels turn. Thought maybe the proportioning valve could be an issue but didn't think I would be able to bleed the rears at all if it was. I am seeing fluid at the rears, not alot with each press but some. Imagine it should be a good purge with each press but it is more of a slight leak, almost like its just gravity feeding it. I'm out of ideas. Even took off the drums and hit the pedal. The new cylinder and pins didn't even move. What am I missing hear. So tired of messing with these brakes. Side note when bleeding last time the front chamber of the brake reservoir, which is for the front brakes I believe, would pop up a bubble while bleeding the rears. Doesn't make sense to me.
 
Some people have reported issues with the combo/prop valve hanging up. There is some old trick to put a C-clamp on it to aid in bleeding or something. You say you bled them, so it sounds like there is fluid coming from the wheel cylinders. How does the pedal feel? It's possible the master cylinder has failed. It's also possible you have a restriction, like some debris is blocking the line. A restriction would cause a stiff pedal and an internal leak in the master would make it soft.
 
Couple questions, have the rear shoes been adjusted?
Was the master cylinder replaced? Left open to drain will work was done?

The shoes will not fit the drum well until the they wear some. This said they should still operate well enough to keep from turning by hand.
With a helper. Have helper pump up the brakes, and press hard, on a high firm pedal. Put a hose on right front caliper. Open bleeder screw, while helper pushes to floor as quickly as they can. Close bleeder. Check to see if rear brake performance improved. You may need to bleed rears some more after recentering proportioning valve.
Test the brake warning lamp switch in proportioning valve. Use an ohm to ground, should be no continuity. With the wire at proportioning valve off provide a ground and check the red brake lamp in instrument cluster it should be lit.
 
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Used a vac pump once and went old school pedal push with an assistant, but also added some speed bleeders. Again don't seem to be getting much to the rears with each push. The whole line to rears has been replaced, so not thinking line debris unless it's at the pro valve. Pedal is real soft.
 
Shoes have been adjusted. Went until I couldn't get the drums on and backed off a 1/4 to 1/2 turn. Master wasn't replaced and the rear res drained out while I replaced the rear lines.
 
Well a soft pedal can only be 3 things:
  1. Air
  2. leak
  3. bad master

It's not uncommon for an old master cylinder to bite the dust while bleeding. The seals end up traveling in areas of the bore that aren't normally used and can be damaged by rust or debris.
 
I went back and read your original post. It does not say if the master cylinder was swapped. New used or the one that was in it.

Soft peddle is usually air in the lines. If you push it down with the engine running, does the peddle stay or slowly move to the floor?
 
Sometimes you have to ask the stupid questions. Has it been bled at all 4 corners?

Somebody else can jump in here. Isn't there an old trick to loosen one of the fittings on the prop valve to let it burp air out? If the MC went dry, isn't "bench" bleeding the MC worth a shot?
 
Bench bleeding is defiantly worth a try. But you can do on the truck.
I put a bread pan under master with a PIG mat, have my helper pump up the brakes, put my wrench on the flair nut, blue paper towel over the wrench and flair nut, then crack the fitting. Close before letting pedal up. Do ea side several times.
 
Yes I bled all 4 corners. Maybe Ill get a new master, they dont seem too spendy. See what that does.
 
New bench bled master put in, no change. When bleeding the front, brake has resistance, feels normal and fluid rushes out. When doing the back they pedal feels light till almost to the floor then almost a little catch at the end of travel and a small amount of fluid comes out. Think its that proportioning valve is partially blocked. All lines are new to the rear, cant image blockage in the line. No kinks in the line, so sharper bends into the diverter line at the rear axle but no kinks. Am I missing anything else.
 
To shift it back you need simulate a failure in the front
Have a buddy pump up the pedal 4 5 presses, hold down like he's about to crash into a wall. You goto right front, put a hose on the bleeder nipple, then open it as far as you can as fast as you can.
This should shift the valve back to normal.
 
By coincidence, I'm presently going through the same thing with my '89 Blazer. A couple of months ago, I changed my front rotors and then found that one of my front calipers was bad so I decided to replace both calipers. I installed Speedbleeders on each caliper and was able to bleed the front brakes by myself with no problems. I didn't touch the rear brakes at the time and all was well. The following week, I decided to take a look at the rear brakes and noticed that one of my wheel cylinders was starting to leak. It wasn't really an issue yet but I decided to replace both wheel cylinders. That is when the fun began.

I installed Speedbleeders on the wheel cylinders thinking that this will be a breeze. When I tried to bleed the right rear wheel, the 1st push on the brake pedal went to the floor as expected. The 2nd push stopped near the top. It turns out that the the piston in my proportioning/combination valve moved so that it then blocked the flow to the rear wheels. I couldn't get it to reset so I decided to replace it with a new one and unfortunately, they are no longer available from GM. So I took my chances with one that I bought off of Ebay. My original prop. valve was the old cast iron version and figured that it was time.

I also bought one of those tools that you insert in to the Prop. valve by removing the brake light switch and inserting the tool to prevent the piston from moving off center. I then proceeded to bleed the right rear wheel again. This time, the brake pedal went to the floor but seemed to have move resistance and I didn't seem to get the flow that I expected. So I then decided to replace my flexible brake lines with Russell stainless steel braided lines. I put the original bleeder screws back in to the rear brake cylinders with thread sealer applied and tried a vacuum bleeder but there was a leak somewhere and all I seemed to suck out was air from the leak.

Next, I am going to try pressure bleeding using my Motive pressure bleeder. The service manual specifies the Kent-Moore master cylinder adapter J-29567. I looked it up on Ebay and found Kent-Moore J-35938 adapters (is part of J-29567). These will insert in to the bottom of the master cylinder and I will then connect them to the Motive bleeder. I'm pretty sure that this will work and will give it a try soon.

Also, on my Blazer, the front of the master cylinder feeds the rear brakes. The rear of the master cylinder feeds the front brakes. Well that was a long but that's my story and I'm sticking to it.
 
Did you try pushing the button, or using the little tool? https://techtalk.mpbrakes.com/combination-valves/combination-valves

HW3350-Normal-2.jpg


I've never used it, but maybe the right approach at this point is no stone unturned?
 
Little update here. New proportioning valve installed. Little tool installed to keep pin in place. Bled all brakes, pedal feels good now, stiff. Start the Blazer and pedal goes soft. Bled the brakes again and again with it running, now it feels almost normal. Still not getting any action in the rear brakes though. Even pulled the drum off and hit the brakes. No movement on the wheel cylinders at all. So I removed the front pad and depressed the piston on the cylinder and pressed the brake and it moved. So I'm thinking for some reason the wheel cylinders are not being depress by the pads, pins, springs, etc. Everything in there is new, is it possible the Schafer wheel cylinder pins i got are too short? Says they fit my year, and seem pretty universal. What else am i missing here.
 
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