98 Tahoe brake pedal goes right to the floor

[RWTrask]

Good Morning:

1998 Tahoe LT 2dr 4x4 350r 122k miles.
Problem: brake pedal goes right to the floor - no leaks - no codes.
OEM Liberty Chevy in Wakefield, MA can't figure it out after 20 days (10-15-2014).
New rotors, drums, calipers, pads, shoes, master and rear cylinders.
Please help, its my only vehicle.

[email protected]

 

[MO Viet Vet]

Then someone at that shop needs to go back to basics. The system is hydraulic, period. The fluid starts in the master and flows down lines/hoses to wheel cylinders and calipers. The hydraulic cups in the master push fluid to the others. The fluid enters the wheel cyl and cups get pushed that move the metal cups and bullets to move shoes. The fluid enters the caliper and the caliper piston seal surface at the square cut seal is what holds fluid back and allows the piston to move the pads. Either one or more of those surfaces/seals is failing or there is air in the system. Or there is a leak and someone is blind. Back of master bolts at booster and may be leaking fluid in to the booster.

 

[RWTrask]

Thank you for responding.

I absolutely agree with you. I was a Mechanical Engineer (retired) and have worked on cars ever since 1962. My hands don't work so well anymore and therefore I have taken my 98 Tahoe to a good friend at Meineke in Reading, MA. There are no leaks, no codes and the brake pedal keeps going right to the floor. After chasing our tail for a week, I carefully drove over to Liberty Chevrolet in Wakefield, MA. They replaced the new master cylinder Meineke installed with an OEM. Liberty worked on it for twenty days and still the brake pedal goes right to the floor.

 

[MO Viet Vet]

If the pedal goes to the floor and if the bleeders are opened and nothing coming out then you need to check and make sure the rod adjustment at the back of the master cylinder is correct. The rod is attached to the brake pedal. If the master is bench bled and flows ok and then bleed at the lines where they attach at the master and the fluid flow is crap then there is where the problem is. Excess clearance at that area.

 

[RWTrask]

We had that same discussion at Meineke and Liberty. The fluid flow is homogeneous - no air.

 

[MO Viet Vet]

Then somewhere there is a weak link in the system. The rod adjustment? Have to watch the caliper pistons move when apply with tires off? remove drum and slowly watch the wheel cylinder bullets move? Correct adjustment at the rear shoes? Remove drum one at a time and have a strong person hold pressure against the 2 shoes and apply and see if pressure pushes or bypasses? Put a flat blade screwdriver between the rotor and pad at piston side and apply and pry back against and does it bleed or keep constant pressure? Guarantee I could figure it out if the vehicle was here. Pressure is either being made and then bleeds off because is bypassing or wrong parts or...etc.

 

[RWTrask]

However, one issue that we observed; We clamped the flexible brake lines, one on each side of the front and one at the rear. When you push down on the bake pedal (ignition on or off) the pedal is normal - no issue. When removing one clamp at a time, only the drivers front unclamped line causes the brake pedal to travel all the way to the floor. No leaks, no air, no codes and no brakes.

 

[MO Viet Vet]

I wish you would have said that earlier. You have a problem at that caliper then. If fluid flow through the hose is ok then that caliper is faulty. Sorry I didn't say something earlier about pinching hoses. The problem is there somewhere.

 

[RWTrask]

I'm thinking that there is an ABS bypass valve that is not seating, relative to the drivers side front brake.

 

[MO Viet Vet]

Anything is possible. Is there another assembly that the lines from the master runs through before going to the lines for each side at the front?

 

[RWTrask]

Yes, the lines from the master go to the ABS and then go to the calipers/cylinders.

 

[MO Viet Vet]

Ok, now we have a whole other can of worms. If I remember, those things are not cheap and the bleed procedure is a pain even if has no problems. Did you look up the bleed procedure on google or check on you tube?

 

[RWTrask]

Yes, the brake lines from the master go to the ABS and then to the front and rear.

 

[MO Viet Vet]

Check on you tube.

 

[RWTrask]

Also, GM Tech stated the ABS was functioning correctly. He also stated that the system was bled several times relative to specifications. The first thing I did when I drove out of there was to find a gravel road and lock up the brakes. The brake pedal still goes right to the floor.

 

[RWTrask]

By the way, Trask Mountain in the Northern Oregon Coast Range, is the tallest mountain in Yamhill County, Oregon is named after my cousin Elbridge.

 

[MO Viet Vet]

I do love living here and drive all over for the views. I went to you tube and found lots of info about the bleeding of brakes 88-98 with the abs module. If the module is ok then the problem must be the caliper. Pinch the line at caliper and hard pedal means the fluid is getting to the line thru the abs module. Caliper problem in my opinion. Good luck.

 

[lesterl]

DO NOT PINCH THE FLEXIBLE LINE. You can collapse the inner liner and then your brakes can lock up on you......

That being said I bet you need alot more bleeding and maybe someone different to bleed it. The ABS is hard to bleed, but can be done with patience.

 

[MO Viet Vet]

I think there is a you tube about bleeding it with a scanner attached during the process. Still think there is a problem with that one caliper.

 

[lesterl]

Sounds like he has air in thar.....