red brake light comes on when braking

[K5dreamer]

ok, so im a little confused on this one. the brakes on the truck (1986 m1009 diesel with hydroboost) were pretty well worn, so i decided to replace them, ive already done the fronts, new rotors and pads, but havent gotten to the rears yet. under gentle braking everything is peachy, however. when im on the brake for a while, or get on the brake with anything above light pressure, the emergency brake light comes on, and stays on, till i tap the brake pedal quickly which seems to "reset" something, and the light goes out. i popped the top off the master cylender, and the fluid level is fine, but i noticed the gasket had the little "domes" pulled inside-out. i popped them back out the way they are supposed to be, and the truck felt much better for about a 100 yards, but then the braking got spongy and the light started coming on again. i know for a fact the rear brakes need to be replaced, but im wondering if the gasket on the master cylender needs to be replaced, or if i have another problem entirly. any help would be really appriciated.

 

[lectric80]

This usually implies a loss of brake pressure. Maybe that a wheel cylinder in the rear is leaking or lost pressure momentarily.

 

[Muddytazz]

Might pull the rear drums and check for leaky wheel cylinders. Sounds like air is getting in to your brake system.

 

[lak2004]

have you bled the brakes at all 4 corners? not sure why the e-brake light would come on tho, is the pedal falling at all for the e-brake? i had that happen on one of my trucks and had to use a rubber band to keep it up, wasnt really noticeable though.

 

[lak2004]

wow 3 answers at once!

 

[Goose]

Red brake light on when...

there is a pressure difference from front to rear.

Like one of the other guys said, prolly a leaking rear wheel cylinder.
So if a wheel cylinder leaks, it creates a pressure drop, which moves the internals of the proportioning valve due to higher presser on the other side (front) and makes the connection for the brake light.

 

[Goose]

For clarification:

The front brake line from the M/C comes into the valve at the dotted line in the diagram between the metering valve and switch. The rear comes in on the other dotted line.

Front brake lines go out at the metereing valve, rear (singe and splits later) at the proportioning valve.

 

[dremu]

ok, so im a little confused on this one. the brakes on the truck (1986 m1009 diesel with hydroboost) were pretty well worn, so i decided to replace them, ive already done the fronts, new rotors and pads, but havent gotten to the rears yet. under gentle braking everything is peachy, however. when im on the brake for a while, or get on the brake with anything above light pressure, the emergency brake light comes on, and stays on, till i tap the brake pedal quickly which seems to "reset" something, and the light goes out. i popped the top off the master cylender, and the fluid level is fine, but i noticed the gasket had the little "domes" pulled inside-out. i popped them back out the way they are supposed to be, and the truck felt much better for about a 100 yards, but then the braking got spongy and the light started coming on again. i know for a fact the rear brakes need to be replaced, but im wondering if the gasket on the master cylender needs to be replaced, or if i have another problem entirly. any help would be really appriciated.

The light coems on for two reasons: one, that the parking brake is engaged, and two, that the combo valve has detected a major difference in pressure between the front and rear circuits.

The gasket on the cover of the master is not the problem and the cones will pop down as the brakes wear and the fluid level drops.

I would bleed the system as it's cheap and simple. If that doesn't do, give the brake hoses a good examination, as well as the calipers and wheel cylinders for leakage. If you see no indication of leakage -- and do the rear drums if they need it -- then the master could be old and worn out.

Also note that originally the military used DOT5 or whatever the silicone stuff is called, but most secondary users have switched them back to DOT3 (regular brake fluid) 'cuz it's a HECK of a lot cheaper. You might well just bleed, bleed, bleed to flush the system out.

-- A

 

[K5dreamer]

wayell crapballs, maybe the rear brake cylenders are leaking afterall. i checked and they looked alright, i figured it was the axle seals leaking all over my rear brakes. i dont suppose all new brakes on the front (rotors and pads) could cause enough of a pressure differential with old worn out rear brake components to cause that light to come on could it??? (i know im being optamistic) guess ill go price out some brake fluid and brake cylenders, and do it all at once, ive been holding off on puttin the new drums and shoes on cuz i didnt want them to get soaked in lube from the axles. id be pretty pissed to put em on only to soak em in brake fluid. although if i had a leak like that, and im driving the truck on a daily basis, wouldnt my brake fluid level be low???? its been doin this for a while now.

 

[kwissman]

You might see if your proportioning valve has a button on the end you can push to reset it. If you only bled the front then the piston may be all the way to one side.

 

[dyeager535]

Sounds like the light keeps coming on, not staying on constantly, if so, the valve is ok.

Make sure of your leaking situation. Big difference between brake fluid and gear oil. You could have two problems here, and end up spending money replacing things that aren't bad. Wheel cylinders you should be able to tell by just pulling the drums, they can't leak from many spots, and the type of fluid/oil is going to be obvious.