The ABS and Brake light came on last week and it feels like I have no stopping power on the snow at all the front tires lock up real easy felt like I have no rear brakes at all.

So I jacked up the back of the car, put it in gear and pressed the brakes and nothing. I know the rear breaks only do 20% of the breaking but you should be able to stall the car with the weels in the air.

So I pulled the E-brake and the car stalls.

I took it in and got the ABS code pulled and it was #51: Rear ABS motor free spins

I also noticed that there is a tiny ammount of fluid seeping down the break booster, that is comming from inbetween the booster and master cylinder. There is still lots of break fluid in the reservoir, and it has never been run dry.

I am leaning towards it being somthing hydraulic because of the rear breaks not seeming to work. What would cause the the rear brakes to do this? Could it be the master cylinder or could it be somthing else?

 

Are both the ABS light and the BRAKE light on? If so your ABS is inactive due to a problem and you also have a problem in the base brake system. If you have a leak between the master and booster, that means the leak seal has been bypassed by fluid and you more than likely have a porblem with your master cylinder. Without inspecting it in person I can't say for sure but you need to get it checked out ASAP, brakes are not something to mess around with.

 

Both the Abs light and brake light are on, and there is a small leak between the master cylinder and brake booster. Also on top of that Im makes a little bit of a hissing noise when you apply the brakes. You can only hear it if you have your head by the brake booster and I am assuming that it is not normal.

So do you think I should just go ahead and change the master cylinder? Because If it is leaking between the two It will have to be changed anyways right?

Also If I do replace the master cylinder when do you bleed the ABS unit? Is it the last thing you bleed?

 

I would asuum that the code set cause you jakced up the rear.



If there is fluid leaking between the master and the booster then the master cyl has blown a seal and should be replaced. this will however be the first one on an fbody that I have ever seen go bad.

if you do deside to do it, pull the old one off. get the new one. usualy have to reuse the old resivour so put that one. now the new one should come with 2 fittings and 2 plastic lines. use these to "bench bleed" teh master before you install it. do this by threding the fittings into the holes that the brake lines go into. now attach the hoses to them and run the other end of the hoses into the resivuir and fill it with brake fluid. put it in a vice and with a screwdriver or puch or what ever. press the piston into teh cyl several times untill all the air is out.

Now install it one the car and hook everything up. now have someone pump teh brakes a few times then have them hold it to teh floor while you crack open one of the brake lines at teh ,aster just enough to let any air and a little fluid out. do this a few times on both lines. after that is done start the car up and pump the brakes and see how it feels. that should be all that you eed to bleed since you opened the system at teh highest point.



now. if you want o be sure, clear the codes and drive teh car and see if the lights come back on. anytime the ABS ligts are on it means that the ABS enable relay has been acivated and the system has been shut down

 

Sounds like it could be the proportioning block in the abs unit, or you have air in the rear lines. try bleeding the brakes starting with the rear and see what happens. just cuz you havent taken off the back lines doesnt mean it cant get air in there, it couldve been in there a while and just worked its way to the caliper.

 

Well I am gonna bleed the brakes tommorow and see If that helps an. I was also wondering what is it that makes the brake light come on. I know it means somthing is wrong with the brake system it self aside from the ABS, but what is it that can trip the light to go off???

 

The BRAKE light just means the parking brake is on. Not too much is monitored in the actual brakes by the computer if any. If the abs unit has failed the computer will know and either turn the ABS INOP light on or throw a SES code.

 

The "BRAKE" light is also typically set by the proportioning valve shuttling completely to one end (or chamber). The valve grounds the sensor wire and turns on the "BRAKE" lamp. This indicates a pressure failure of one circuit (front or rear). The valve will move in response to the loss of pressure.

Most people think the lamp is only an indication of the E-brake position.

 

You are wrong. The BRAKE light comes on if the parking brake is on, you have a low reservior volume, or if the pressure differential switch is tripped. Basically if you have a BRAKE light on you have a base brake problem that needs to be taken care of. The ABS light comes on if there is an ABS problem. If the light just come on occasionally or flashes it means there is an ABS problem, but it is not severe enough to disengage the ABS. If it stays lit all the time it means you have a major ABS problem, and the ABS has been bypassed.

 

I think you are thinking about the pressure differential switch. All the proporting valve does is limit the pressure to the rear brakes during hard braking so the rear of the vehicle doesn't come around.

 

I know on the older stuff, the proportioning valve was indepedent from the master cylinder and the brake light connector was on the PV. The valve plunger would actually make contact with the pin and supply a ground path for the lamp.
The newer models ('71 up) integrated the PV into the master cylinder.

The proportioning valve actually proportions fluid between the front and rear brake line circuits. If there is a break on either circuit, the valve will seal that circuit to maintain fluid pressure for the remaining circuit.

 

So if this happens then how do you go about opening the valve so that both the front and rear brakes are operational.

This would explain why I am getting no back brakes at all. So when I try to bleed the rear breaks then no fluid will come out?

 

I recall reading that to reset the 'brake' light after the prob is fixed, it takes a hard [400 lb?not sure of amt but a lot!] application of the brakes..
Leaving for work now, but if I remember tomorrow I'll post more from the service manual..

 

I usually just open one of the front bleeders and give the pedal a quick firm push to try to make the PV reposition (re-open the rear path).
I have had to remove the lines before and physically move the valve with a screwdriver.
Usually the first method works well enough to get the lines bled.