Help troubleshooting brake issue

[argonaut]

I'm trying to get my 72 Blazer road worthy for the trip home on Friday. The brakes don't seem to feel like other trucks with power brakes.

WHen I apply the brakes the pedal travels half way down and seems to apply maybe half or so of the overall brake pressure. Then even if I STAND on the brake pedal it doesn't seem to move much or apply much more brake force.

I tried removing the vacuum line from the booster, plugging it and driving/applying the brakes. The function was similar but seemed to have to push even harder on the pedal.

I then hooked a different hose to the booster and applied a vacuum with my mouth and it seemed to hold the vacuum. SO I reconnected the vacuum line.

I've also bled all the brakes and flushed with new fluid.

Could this be a bad booster, or an issue with the prop valve? Help!

[indianajoel]

Drums in back and discs up front? I wonder if the drums could be out of adjustment.

You checked the vacuum coming to the booster? Those check valves in that line can fail, i think.

Are the pads and rotors new or at least checked out? Slider bolts slide nicely?
That can cause a soft pedal if they get rusted stuck.

It's not hard to test the booster. It's in every Hayne's manual and good to know.... If you apply pressure with the engine off, the pedal should go part way to the floor and then be firm, then you start the engine and the pedal should drop a little more. And then there's some similar check you do with you foot on the pedal, engine running and then you kill the engine. On my trucks no change occurs when I do the latter test.

Or could you have bled the system wrong and missed a step? On a '90's S-10 you're supposed to tap the front pistons to free air bubbles and wait 15 seconds between pumps (for the ABS). Toyota's load proportioning valves need to be bled. I wonder what brakes went on your truck. But my 67 w/ power brakes didn't need special bleeding.

good luck

Joel

[cparman]

back to the basics!
pull wheels and inspect what you have. Do not assume anything.
when you pull the rear drums, use your thumb and peel the rubber boots on the wheel cylinders back a bit and see if any fluid runs out. If it does it is time for wheel cylinders.
You might be able to get a decent pedal even if you have the above problem, but adjusting the rear brakes properly, and bleeding the brakes. This might get you home, but don't fool around with bad brakes. If you are not worth that your Blazer surely is. My guess is that the rear brakes are out of adjustment, and that there is air in the system. Bleed and adjust to get it home and then FIX IT RIGHT!

[argonaut]

The Blazer has front disks/rear drums. I jacked up the rear and had my cousin step in the brake pedal to find out that the rear brakes were not being applied!

I checked all the wheel cylinders and there was no corrosion, movement was good. Adjusted the rears properly and it felt a bit better. But still not getting positive braking in the rear.

I then moved up stream to the proportioning valve, thought maybe it wasn't allowing pressure to the rear. However the sensor was not tripped. I removed the sensor to see if the valve was centered. it looked like it was as far as I could tell.

I bled the entire braking system again.

The application of the brakes feels different depending on how hard you step on the pedal. I'm now thinking perhaps this is a bad master cylinder since I've checked almost everything else.

I still have teh remaining questions:

How can I be sure that the prop valve is not cutting off pressure to the rear?

Is it possible for the master cylinder to be going out and not applying much braking force to the rears while still applying some to the fronts?

[cparman]

Does it feel like you have half of a brake pedal? If the prop valve is tripped, USUALLY there will poor fluid flow to the rear wheels when trying to bleed the brakes. If you bleed the fronts, and get a really good squirt, and then bleed the rears, and it kind of dribbles out, I would think the prop valve is tripped. Now that you have the rear brakes adjusted, I might suggest stomping the hell out of the brake pedal a couple of times, and then try and bleed the rear brakes again. Sometimes this will center a stuck prop valve. When I bleed the brakes on 60's-70's vintage stuff, I will usually do it this way. Have your pumper, pump the brakes 3-4 times and hold it down, open the bleeder, and repeat until you get good clean fluid and a nice squirt. If your ride has been sitting for a long time it could be a bad master cylinder, but think some patience, and a good pumper will get the brakes feeling good again. I know you will figure this out, but it is frustrating as hell when you are in the middle of it!

[argonaut]

good advice, yes it does feel like I have half a pedal, then a hard pedal.
I tried really stomping on the pedal but that didn't work, so I'm thinking that maybe the prop valve is stuck but somehow not triggering the sensor. i think my only option at this point is to disassemble the darn thing.

[cparman]

I read here where some folks will push the pin back in with a small C Clamp and then try and bleed the brakes?? I have never done this, but understand what they are doing. There is a small rubber boot at the front of the prop valve. If you remove this, you will see this pin. Push it in, and hold it there. See if you get a pedal, and then release the pressure on the pin and see if you still have brakes. HTH

[argonaut]

I believe this section view of a prop valve is correct for our trucks:


My button on the end didn't really stick out, and when I try pushing on it with a small allen wrench it just feels like its under spring pressure, doesn't really move much.

So I pulled the sensor out and looked down in to see if the little shuttle piece was offset to one side, which would indicate that the valve needed to be reset. What I found was that the little indent was right in the middle where it should be. This lead me to believe that the prop valve was not the root cause of my problem.

[indianajoel]

Yeah, maybe you should turn your attention away from the proportioning valve.

I like cparman's thought on bleeding the rears and looking for good fluid flow. Then you can rule out the prop valve as a problem.

Are the rear wheels not locking up when the pedal is applied? With these dual diaphragm master cylinders one circuit is rear and one is front brakes, right? If the prop valve checks out good and so do the shoes and the system is free of air, then maybe the master cylinder diaphragm for the rear brake line is worn out. A new one must only be 10 or 20 bucks, right?

[cparman]

Sometimes when I am up to my neck in a problem child like this, I will have the truck jacked up on a lift, or at least all 4 wheels off the ground and the truck level, and open both rear bleeders, and just let it drip. Run at least a half a pint of brake fluid thru the rears, and then do the same with the fronts. I have pumped the brakes so much on some cars trying to get a pedal that the fluid would actually be foaming from all of the pumping, causing air to be in the fluid. Gravity bleeding ( the above) works! If you are getting decent fluid flow out of the rears then the prop valve is not the problem. My dumb question of the day is: You have adjusted the rear brakes to the point that you can hear the brake shoes rubbing slightly on the drum, but not causing so much drag that you cannot turn the wheel?