Son in law brought his blazer to me with a poor braking problem. On inspection the master cylinder (front 1/2) was low on fluid, there is a soft pedal, the brake light is on and the ABS light is off.

After adding fluid (condition remains the same) I started trying to isolate the leak. Sequentially I bleed each outlet at the master cylinder, at the inlet to the ABS controller and the outlet of each ABS controller. I could not get the outlet from the ABS controller to the rear brake lines to bleed any fluid.

Not sure of the next step. How do I determine if the master cylinder is leaking internally or if the ABS unit has a plugged valve?

Thanks in advance

Kevin

 

I am not 100% sure but I think I have read somewhere that a suitable scan tool is needed to properly bleed the ABS system if it runs dry, could be that the valves need to be manually opened? Maybe someone else on this forum with a bit more knowledge can clarify it a bit more?
I changed the brake fluid on mine a few months ago but I made sure the master cylinder never ran dry & I had no problems.

 

Dave,

Your correct that there is an ABS Bleeding procedure accessible via a scan tool. I have a scan tool and have run the procedure - it will firm up the brakes during the procedure and for a minute or so afterwards.

There are no ABS codes - the ABS thinks it is fine. I do have a brake light indicator lit and have confirmed it is lit by the sensor in the ABS body that measures hydraulic pressure differences between the front and rear.

I just can't find a leak!!! This is driving me crazy.

It does seem that I should be able to get fluid from the valve body to bleed when I loosen the rear brake line at the valve. Doesn't it??

 

im wonder if the proportioning valve closed on you?

 

The proportioning valve is called a combination valve in a kelsey-hayes 4wal system - 1998, per alldata.

I found another 98 blazer. It bleeds freely from the ABS output to the rear wheels connector. So is there some way to reset the combination valve?

 

the proportioning valve is not part of the ABS. the proportioning valve will close either the front bakes lines or the rear if the pressure changes meaning if the rears leak the pressure changes so the valve will close the rear brake lines off. if that makes sense? here a pic of it usually it on a bracket under the master cylinder More Information for ACDELCO 12389521
your ABS module might have a brass looking pin sticking out might be under a rubber cap try pushing it in those can also close off and stop fluid flow.
not sure about your 98 but my 96 had this prob and thats what i found

 

I found the problem. Rear brake lines leaking, proportioning/combination valve actuated, will not reset without adequate rear brake pressure.

To find the leak I disconnected the rear brake line from the ABS valve. put in a plug. hit the brake pedal a couple of ties with a 3lb hammer. brake light goes off. reconnect the rear lines and pump the brakes until you find the leak.

@Scott - thanks for getting me to look at the combo valve again.

Kevin

 

I spoke too soon. After replacing the broken line I cannot get fluid past the pro portioning valve. I'm guessing I need to have some type of tool to hold the pro portioning valve open until the pressure is built up.

Any suggestionson how to bleed this system?

 

pressure bleed from caliper
I have a mightyvac vac pump that I can also pressurize
I would try pushing the fluid backwards with it
Mityvac Hand Vacuum Pump Kits

 

BTW
Did you check the bleeder screw isnt plugged ?????