Bad Master cylinder or Brake Booster? 96 S10 Blazer

[Chevy Nova]

I just finished replacing all the brakes on my sons 96 S10 Blazer. New calipers wheel cylinders the main brake line from front to back the flex line joining the main line to the rear wheel lines as well as the rear 3/16" lines. No leaks. The pedal however doesn't seem right. It is the same as before the brake job. It doesn't seem like it is quite right. I haven't driven it that often so Im not sure if it is working like it should. Here is the problem. If I were to try and lock it up going forward really any speed it stops but seems slow in doing so. Its like it isn't using power assist. I know it is if I turn of the truck after about 1 push on the pedal it gets real stiff. If I turn it off while moving I can stop it but with a lot of effort. I did the test turned of the motor, pumped the brakes until they got stiff, left my foot on the pedal and my foot drops a little. Sometimes while its pushed down like that I hear a continuous hiss. Not every time. I checked the grommet into the power booster its ok and the check valve works one direction as it should. Im thinking the proportional valve is stuck for only the front brakes or the power booster is on its way out. There is a hiss when you first put on the brakes and a hesitation to brake while thats happening. I do hear a idle change with my foot on the brake and even more so when I start it with my foot on the brake.

1996 S10 Blazer 4x4 4 door 4.3 Litre

Any help would be appreciated.

[brcidd]

Is brake light on? A stuck proportioning valve will set the brake light on. You can also force a reset of valve by removing either end caps and pressing the proper direction to center it.

A constant hissing is a bad vacuum booster.

Sometimes when brakes get a leak- the pedal goes down, which also puts the master cylinder plunger in an area it has never been before- longer stroke- and now the corroded plunger has worn/torn the o-rings and caused an internal leak. This is why when ever I bleed the brakes- I put a 2x4 behind brake pedal- to keep the plunger in the master cylinder in its previous working range- before the brake leak.

Just some thoughts.

[Chevy Nova]

brcidd
No Brake light on. I did pull the little black cap on either end and nothing was protruding further front to back? Coincidentally the Air Bag light came on. Not related Im sure. It did go off after a few starts. Never lost the pedal except when I ruptured the brake line the brakes feel the same as before I replaced everything. I know they are not right. Maybe I will raise the rear end and try the brakes when the rear wheels are up and spinning to make sure the rears are working.

Good idea about the 2x4 though. I think my next step will be to remove the brake lines from the master cylinder and put in plugs and see how the brakes feel. See if they feel more firm. If they do I can put the lines back on 1 at a time and see if the front or rear are affecting it. If I dont notice a difference I guess I will replace the Power Booster. The master cylinder can be the last. Should you be able to lock up the brakes and stop the truck and maybe hear chatter from the ABS? Or should it be a gradual firm stop?

I appreciate the help.

[Chevy Nova]

Bump

Anyone else have any ideas?

[rhandwor]

Look at the calipers on the front the bleed screws have to be on the bottom. If they are on top you can't get the air out of the system.
If you have anti lock you may need a scanner to bleed system.

[Chevy Nova]

I believe the air is out of the system I gravity bled them and then bled them at each wheel. I think I have to rule out the master cylinder as well as the brake booster. I get different results from the brake booster. I will turn off the truck pump the brakes, maybe 1 and a half times and it gets firm. Hold my foot on the pedal, start the truck and sometimes my foot drops and I hear a steady hiss. Sometimes there is no hiss. Once or twice the pedal barely moves. No fluid leaks. The check valve is good in the booster. If I wait a minute or two and pull the hose to the booster I still hear vacuum escape. The more I pump the brakes though the rpm does fluctuate. Im leaning to the booster being bad. Im not sure why I would still have vacuum at the check valve if the diaphragm was bad. Wouldnt it disappear within a few minutes?

[rhandwor]

Unplug the booster hose and put a bolt in the hose to stop the vacuum. Then check the brakes.
If brake fluid is leaking between the booster and master cylinder replace the master cylinder. Look for deposits on the engine side of the cylinder. Inspect the hoses to the calipers for bulges.

[Chevy Nova]

Pulled the hose to the Booster pluged it and brakes pump up hard after the vacuum is gone. Im going to try and bleed the brakes again and see what happens. Im leaning to the booster because of rpm change.

[rhandwor]

If you replace the booster measure length of rod before returning for core charge. Place a flat piece against next to rod and measure the length. The rods are adjustable.

[Chevy Nova]

rhandwor
Checked the old against the new they were the same. The only thing weird was the new power booster had no gasket between the firewall and the booster. I called Autozone to make sure it wasn't missing.
Bled the rear brakes today and adjusted the rear a little tighter. Didn't seem to help much. I did notice the more I pumped the brakes the more the RPM fluctuated so I decided to replace the brake booster. For anyone's information you can do it without disconnecting the lines from the Master Cylinder. Just have to be careful moving it out of the way. The nuts on the inside holding the brake booster (4) are a pain but can be done with a 3/8" drive 15mm deep socket and a universal. The lower right has to be a box end wrench and 15mm shallow socket after the nut is out far enough. The hardest part was putting the nuts on the two top studs. Large hands dont help. Tape one of the nuts to the deep well socket. Just enough to prevent it falling off when you get it up there.

I dont think the master cylinder is bad there was no fluid in the booster I think I would have seen it my the shaft that pushes on the plunger in the master cylinder. It was rusty. No sign of fluid.

It helped some no fluctuation in rpm. Breaking a little better. I will bleed the front tomorrow. I was hoping it made a huge difference. I wish I knew what these were supposed to brake like.

[rhandwor]

When I said to look at calipers bleed screw being lower if mounted on wrong side the bleed screw will change from top to bottom. It will be impossible to bleed properly.

[Chevy Nova]

rhandwor
I was referring to the power booster. The rod was the same length. The calipers are correct. Thanks. Im going to bleed the fronts again right now as soon as my assistant gets out of bed.

Just to be sure I went out and checked on the bleeders. They are up by the flexible hose connection at the top. I dont think the hose would be long enough to put them in the other way.

[rhandwor]

http://www.ebay.com/ Search item 331803658672
Remember sitting in front seat right front is the passenger side.
Look at picture.

[Chevy Nova]

rhandwor
That is a newer style caliper. Im sure the calipers are correct. Its all good now.


Bled the front brakes today both 2 person and gravity. Brakes are much better. The brake booster made a big difference. For those people hearing a big hiss when applying the brakes for a 1/2 or 1 second. It appears that the booster is starting to go. Found a vacuum line that was softened by oil or brake fluid and replaced that. It was the one down to the trans. It now shifts better as well. Thanks to all those that replied. Hope this helps someone else along the way.

[rhandwor]

Glad you found the problem.