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-   -   90 Chevy Truck Brake Pedal sinks after stop. (http://www.hotrodders.com/forum/90-chevy-truck-brake-pedal-sinks-after-stop-202033.html)

pemdj 07-15-2011 09:38 AM

90 Chevy Truck Brake Pedal sinks after stop.
 
I have a 1990 Chevy c-1500 2wd, Auto, 4.3 engine with power brakes and ABS. After coming to a complete stop the brake pedal travels another inch or two towards the floor. I installed a new Master Cylinder and bled the Brakes. The new master cylinder is doing the same exact same thing as the old Master Cylinder. Any advise ???

Techtom2001 07-15-2011 09:14 PM

Quote:

Originally Posted by pemdj
I have a 1990 Chevy c-1500 2wd, Auto, 4.3 engine with power brakes and ABS. After coming to a complete stop the brake pedal travels another inch or two towards the floor. I installed a new Master Cylinder and bled the Brakes. The new master cylinder is doing the same exact same thing as the old Master Cylinder. Any advise ???


Try unplugging you ABS fuse and see if that makes a difference. It is also possible that you a pin hole in a line somewhere.

302 Z28 07-16-2011 09:39 AM

Possible the power brake booster is bad. Is your brake pedal excessively hard?

Vince

MouseFink 07-16-2011 10:10 AM

ABS modulator
 
You must bleed the ABS modulator then bleed the brakes in the usual manner. You cannot bleed the ABS modulator without cycling it with a GM Tech 1 scan tool. Those scan tools can be found on e-bay for $300 to $400. You also need two OTC 7853 proportioning valve depressor clamps to use on the ABS modulator accumulator valves as the ABS unit is cycled. If you do not correctly bleed the ABS modulator, you can pressure bleed or foot pump ten gallons of fluid through the system and never get any brake pedal.

The shade tree method of bleeding the ABS modulator is to get on a gravel road and get up to 50 or 60 MPH and slam on the brakes and slide the tires. That may cycle the ABS modulator. The problem with that is it takes guts to get up to 50 or 60 MPH on a dirt road without any brakes.

pemdj 07-18-2011 09:26 AM

Quote:

Originally Posted by 302 Z28
Possible the power brake booster is bad. Is your brake pedal excessively hard?

Vince

The brake pedal is spongy.

MouseFink 07-19-2011 10:13 PM

Abs
 
If you have bled the brakes properly, your ABS modulator still has air in it.

pemdj 07-20-2011 08:53 AM

The ABS Hydraulic Unit ( Isolation/Dump Valve ) was stuck in the bypass position causing a constant internal leak. I replaced the ABS Hydraulic unit , bled the brakes and now they work perfect. I was told that this is a common problem for these trucks.

pemdj 07-20-2011 08:54 AM

Quote:

Originally Posted by Techtom2001
Try unplugging you ABS fuse and see if that makes a difference. It is also possible that you a pin hole in a line somewhere.

Bad ABS Hydraulic Unit

pemdj 07-20-2011 09:06 AM

Quote:

Originally Posted by pemdj
The brake pedal is spongy.

The ABS Hydraulic Unit was Bad

pemdj 07-20-2011 09:13 AM

Quote:

Originally Posted by MouseFink
You must bleed the ABS modulator then bleed the brakes in the usual manner. You cannot bleed the ABS modulator without cycling it with a GM Tech 1 scan tool. Those scan tools can be found on e-bay for $300 to $400. You also need two OTC 7853 proportioning valve depressor clamps to use on the ABS modulator accumulator valves as the ABS unit is cycled. If you do not correctly bleed the ABS modulator, you can pressure bleed or foot pump ten gallons of fluid through the system and never get any brake pedal.

The shade tree method of bleeding the ABS modulator is to get on a gravel road and get up to 50 or 60 MPH and slam on the brakes and slide the tires. That may cycle the ABS modulator. The problem with that is it takes guts to get up to 50 or 60 MPH on a dirt road without any brakes.

IT WAS A BAD ABS HYDRAULIC UNIT

pemdj 07-20-2011 09:14 AM

Quote:

Originally Posted by 302 Z28
Possible the power brake booster is bad. Is your brake pedal excessively hard?

Vince

IT WAS A BAD ABS HYDRAULIC UNIT

MouseFink 07-20-2011 09:27 AM

Hard to find ABS
 
I had to bleed my 1991 S10 Blazer brakes with 4WAL ABS brakes (EBC-4) and I tried to find a ABS 4WAL EBC-4 modulator assembly. No one had one for my S10 Blazer except NAPA and they only had one at a warehouse in Ohio and it cost $800 + my old unit. I checked with Cardone through Autozone and the Cardone represenative told me thay could rebuild my old unit for the same price. The parts suppiers who rebuild 4WAL EBC-4 ABS modulators don't have any cores to rebuild. Tyhat is because when an owner of a 1988-1991 S-10 Blazer has a ABS modulator that is bad, they usually just junk the whole vehicle rather that pay $800 + labor for a rebuilt ABS modulator or they will removed the ABS unit and convert the S10 Blazer to a conventional braking system.

I chose not to install a expensive rebuilt ABS unit so my associate foot bled the brakes in the conventional manner. It took about 6 hours but we finally got good brakes. Apparently using two of those OTC accumlator valve depressors along with a lot of patience got it done. It would have been easier with a pressure bleeder but most car owners would not buy a pressure bleeder that they would not use but once. Most people don't keep a car more than five years.

If you own a S10 Blazer or any vehicle with EBC-4 4WAL ABS brakes, it is important that you keep clean brake fluid in the system and change regularily because the ABS modulator is sensitive to dirt and foreign particles.

pemdj 07-20-2011 09:41 AM

Quote:

Originally Posted by MouseFink
I had to bleed my 1991 S10 Blazer brakes with 4WAL ABS brakes (EBC-4) and I tried to find a ABS 4WAL EBC-4 modulator assembly. No one had one for my S10 Blazer except NAPA and they only had one at a warehouse in Ohio. It cost $800 exchange. I checked with Cardone through Autozone and the Cardone represenative told me thay could rebuild my old unit for the same price. The parts suppiers who rebuild 4WAL EBC-4 ABS modulators don't have any cores to rebuild. Tyhat is because when an owner of a 1988-1991 S-10 Blazer has a ABS modulator that is bad, they usually just junk the whole vehicle rather that pay $800 + labor for a rebuilt ABS modulator or they will removed the ABS unit and convert the S10 Blazer to a conventional braking system.

I chose not to install a expensive rebuilt ABS unit so my associate foot bled the brakes in the conventional manner. It took about 6 hours but we finally got good brakes. Apparently using two of those OTC accumlator valve depressors along with a lot of patience got it done. It would have been easier with a pressure bleeder but most car owners would not buy a pressure bleeder that they would not use but once. Most people don't keep a car more than five years.

If you own a S10 Blazer or any vehicle with EBC-4 4WAL ABS brakes, it is important that you keep clean brake fluid in the system and change regularily because the ABS modulator is sensitive to dirt and foreign particles.

Thanks for the information. I was lucky, my 1990 1/2 ton truck only has Rear Wheel ABS and the unit only cost $ 119.00 at Kragen/Oreilly,s Auto Parts.

MouseFink 07-20-2011 09:54 AM

ABS working or not?
 
I have never tested the 4WAL EBC-4 ABS system on my S10 Blazer since we bled the brakes. I don't even now if the system works because I never drive on wet or icy roads or on a gravel road where they could be tested. The EBC-4 braking system is supposed to do a "self-test" where you can briefly hear the motor run after you start the engine and drive a few yards but I have never heard my unit. The EBC-4 ABS 4WAL modulators are failure prone due to an internal relay that cannot be serviced. If the relay fails and sends continuous 12V to the ABS pump motor, the motor will continually run for a hour or two and burn out. I don't think you have a internal relay for a pump motor on RWAL brakes.

All I know is that I have a good brake pedal that will pass annual state insepction and that is good enough. When the state inspection stations inspects cars, they don't take the car to a skid pad and test the ABS on wet pavement.


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