another brake pedal question

[chris989]

Hello everyone,

I have searched the post and there are a lot discussing soft brake pedal after installing brakes. They all discuss the obvious.

I have a 89 suburban 3/4 4x4.

I installed a new master cylinder last week, bled the brakes and all was perfect. Strong pedal, great response.

a few days later i installed front pads, I did not break the seal on any brake lines, I used a clamp and compressed the caliper piston back into the caliper. Now i have super soft pedal.

My two thoughts are;
I damaged the master cyl. by forcing fluid in the reverse flow or,,
one of my "original" flexible brake lines at the wheels is ballooning slightly, absorbing the line pressure and i can not see it.

Has anyone experienced this problem?



I have done this many times before over the years with no harm to any other components.

[truckmannorth]

I would sugest bleding the brakes. I have seen times when pushing calipers back they suck air in. At the shop we always crack the bleders when pushing pistons back a must on cars with antilock brake as you dont want to push "crud" from the calipers into the antilock valves.
Just my 2 cense
Dale

[chris989]

Hello Dale,

I bleed a lot out of the system and I do not detect any air. I replaced the master cylinder again yesterday think maybe it was damaged but the out come is the same. I bled the master before hooking up the lines, and then bled the lines some more.

I took it to midas and they shrugged their shoulders. They did not have the accessories for a power bleed so I went to brake masters, They too did not know, but did not have time to service the truck..

I will drive it some and then bleed it some more in a day. I just think if it was air i would be able to build brake pedal after pumping but the brake height never changes

Thanks,
Chris

[James McClure]

Chris, Have had similar problems with C-K trucks before. Try this, remove the caliper and carefully pump the brakes a few times. Try to get the piston out of the bore 1\2 to 3\4's of an inch, then push the piston back in. Do this a few times to each caliper. While the calipers are off lube the caliper support "O" rings in the pin holes (4) per side. Do the pump & press operation 1 side at a time. Let me know, jim

[chris989]

James,

I replaced the calipers today instead. I even bought a bleeder.

There is zero change in the system and I am confident there is no air in the system,

I checked the rod from the booster to the master and there is minimal play. It is actuating the master correctly. The only thing i can think of is the proportioning valve. It does not have a bleeder, but i cracked the flare fittings to bleed just in case.

There is a button on the left side but it does not appear to be sticking out. I am not sure of the proper set height?? I have read this need to be depressed to properly bleed the system???? Does anyone have first hand knowledge of this?

I am amazed at how many post there are on the web about chevy brakes but none seem to come to a conclusion..
If I can find a good brake shop i will just take it there.



All help is greatly appreciated.

Jon

[James McClure]

Jon, button on prop-valve should be in normally. if it pops out valve isn't centered and light will be on. When you bleed system do you get a good squirt at every bleeder? Double check hose conditions (all 3 hoses). jim

[chris989]

I believe my brakes are solved,

I put in a new booster, The last and only part available to replace. While installing it, I noticed the interior firewall rubber boot is not there. I adjusted the pedal a little higher (two turns of the threaded rod) and now my pedal is high, firm and quiet.

I was surprised what one turn of the rod would do to the pedal height.

SO ,, i am glad i have all new brakes , but if you have this problem take a close look under the dash first. It might save some time and money.


Jon

[RastaRobb]

That is great you figured it out, but man that sucks. Oh well, brand new brake system will probably out live you.

[LostMy65]

I have been fighting my brakes for weeks.
Stiff pedal until I start the engine - then it goes soft.
My next test will be to plug the Master Cylinder ports and see if I can keep a stiff pedal even with the engine running
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[chris989]

no expert but if your pedal is stiff (motor off) and then soft.

THe stiffness is most likely the resistance with the brake booster and then with engine vacuum the booster assists. (working correctly).

if your pedal is soft while engine running, you must have air in the lines or rear brakes need adjusting???

start with the bleeding,

chris