77 k20 brake issues

[buddeboy89]

So to start off I have a 77 k20 with stock brakes and I'm having a little trouble. Here's the story. I bought the truck and drove it for a couple months and I could feel there was something wrong with the brakes. Well then I needed to use my car trailer and wanted to make sure that all the brakes were working so I jacked it up and checked the brakes and only the front brakes were working correctly. So I checked for fluid at the rear and it was barely coming out so I decided to replace the lines from the proportioning valve to the rear brake splitter. I tried to bleed the brakes after that and the rear still wouldn't work. So I went to O'Reilly's and was talking to them about the issue and I could hear a little hiss of air coming from the brake booster when I depress the pedal. They said that it sounded like a bad booster so I replaced the booster and master cylinder since they came as one. After replacing them I tried to bleed the brakes and they wouldn't bleed at all. The pedal goes all the way to the floor when the truck is off and it will build up just a tiny bit but then I noticed brake fluid oozing out of the top of the master cylinder with the cap on. When I tried to turn the truck on and pressing the brakes the pedal would go all the way to the floor and very slowly return and would never build up pressure. I then replaced the front calipers because the bleeder valves were so junk they wouldn't open. The last thing I can think of is a bad proportioning valve. Honestly what I think is that the proportioning valve is bad and now when I press the pedal it seems like the brakes are pulling it from one side of the reservoir and when it's returning it's returning to the other side of the reservoir which is forcing the fluid out of the top. I would greatly appreciate any assistance with this issue.

[bd]

Is the brake warning lamp on in the dash?  Sounds like you dug yourself deeper into a hole by throwing parts at it without a proper initial diagnosis.  Now, the correction is compounded.  The original failure was probably due to a tripped pressure differential valve, because either the brakes previously had been improperly bled or there is/was a fluid leak.  The combination valve did its job by isolating the system half exhibiting low pressure.  At this point I recommend that you replace the rubber hoses connecting to both front calipers and the rubber hose jumping between the frame mounted rear brake line and rear axle housing tee (if you haven't already done so).  Check the rear wheel cylinders for any evidence of leakage past the piston cups - if either is leaking, replace both.  Properly adjust the rear brakes.  Then remove, bench bleed and reinstall the master cylinder using fresh DOT 4 fluid from a newly opened container.  At this point, loosen in turn the two steel lines coming from the master cylinder where they connect to the combination valve and gravity bleed the lines - it shouldn't take very long - be careful that the master cylinder reservoir never bleeds dry.  Next, try to recenter the pressure differential valve, which is an integral part of the brake combination valve (image) located on the forward cross member.  You may have some success in this by hammering down rapidly and repeatedly on the brake pedal with your foot to try and shock the valve into its centered position.  Gravity bleed the remainder of the brakes, first left rear, then right rear, then right front, then left front - making sure to maintain adequate fluid level in the reservoir all the while.  Bleeding can be performed following the procedure in BLEEDING THE BRAKE HYDRAULIC SYSTEM as well.  Be patient when bleeding the system and have adequate fresh fluid available.  Once the pressure differential valve is recentered, bleeding should proceed with little difficulty.  If the master cylinder continues to push fluid out from under the cap after successfully bleeding the system, return it for warranty replacement.

[buddeboy89]

I went to the junk yard earlier today and found an operational combo valve and put it on and bled it and it had the same issue. The warning light wasnt on and hasn't come on. But who knows if it works either. Do you have to bench bleed the MC first? N is there a special way to bleed the brakes with the combo/proportioning valve?

[bd]

Bench bleeding the M/C prior to installation will save you hours of headache and misery.  Use the procedures posted previously.

[75gmck25]

Also go back and check for small leaks around any of the fittings on the master cylinder or wheel cylinders.   

I recently replaced my master cylinder, and even after bench bleeding it was difficult to get a good pedal during final bleeding.  After cleaning up all the fittings very carefully, I found that one line on the master cylinder appeared to have a shiny area around it that appeared when pressure was applied.  It was so small that I was not even sure it was a leak.  However, after unscrewing and retightening that fitting (even a little tighter than I was comfortable with) I finally got good pedal feel and was able to do a proper bleed.   There is a lot a pressure in the system, and any improperly tightened fitting could make it impossible to bleed.

Bruce

[buddeboy89]

Sooooo. Finally figured out what was wrong. I bought a brake booster that came with the master cylinder already on it. What it ended up being was the rod that goes from the booster to the master cylinder. One that was too short was installed from the factory. Just goes to show you gota make sure everything comes correct.