Brakes - why does the light come on?

Hi All,

I have a '92 XJ I-6 auto with 170K miles. It has anti-lock brakes. When I press on the brake pedal it sometimes feels spongy. If I keep a steady light pressure on the peddle seems to drop somewhat and the "Brake" red light on the left dash cluster comes on. A quick pump makes the light go off. I don't notice the problem under steady firm pressure, just light pressure on the pedal.

My question is: what does the "Brake" light indicate? Does it mean that one of the master cylinder pressure chambers isn't holding pressure as well as the other...?

I've ordered a rebuilt master cylinder that I intend to put on to see if it cures the problem. Any advice?

Thanks
Mike

mhead said:

Hi All,

I have a '92 XJ I-6 auto with 170K miles. It has anti-lock brakes. When I press on the brake pedal it sometimes feels spongy. If I keep a steady light pressure on the peddle seems to drop somewhat and the "Brake" red light on the left dash cluster comes on. A quick pump makes the light go off. I don't notice the problem under steady firm pressure, just light pressure on the pedal.

My question is: what does the "Brake" light indicate? Does it mean that one of the master cylinder pressure chambers isn't holding pressure as well as the other...?

I've ordered a rebuilt master cylinder that I intend to put on to see if it cures the problem. Any advice?

Thanks
Mike

Click to expand...

You're heading down the right road with the MC. It sounds like it is leaking inside (bypassing) or outside you can't see. Look at your carpet right below the MC push rod. It leaks behind the carpet sometimes and you don't even know it.

mark
orgs mfg

Bleeding the Hydraulic Control Unit (HCU) on your Teves MK-IV ABS, which is downstream from the Master Cylinder, will require some method of operating it's valves -- DRBII or equiv tool...

The factory procedure is to bleed "like a normal system" then run the DRBII bleed protocol, then "bleed like a normal system" again -- The idea is to ensure that you do not have any air bubbles lodged in the HCU -- a few bubbles in there can really mess with the ABS - it'll end-up with it's own spongyness!

I've see guys have pretty good luck by bench bleeding the MC after it's installed leaving it full, then connecting the hydraulic lines and applying a small amount of vacuum to the MC's intake ports at the bottom of the reservoirs -- drawing any air introduced backwards out of the system (too much vacuum could damage the MC) - then proceeding with a "normal" power bleed --

While not the "approved" factory protocol, that should minimize the risk associated with pumping that air bubble into the HCU and running a 4-corner power bleed will keep things moving fast enough so a bubble will have dificulty stoping in the HCU and causing issues...

Bleeding

They got it right. Buy yourself a Mity-Vac with the bleed attachment and go for it.