Brake Proportioning valve Question
 

My buddy was having some issues with his brakes, so I helped him dig into it to diagnose the problem. Seems he had a rear brake caliper piston that was leaking brake fluid, and it set off the brake warning light triggered by the proportioning valve. When I opened up the master cylinder, the reservoir for the rear brakes was full, and the reservoir for the front brakes was dry, much to my surprise. I was expecting to see the reservoir for the rear brakes dry since the leak was in the rear. And yes, I verified which reservoir went to front and rear brakes when I retracted the rear caliper pistons and saw the fluid level rise in the reservoir that was already full. I had always thought that the prop valve only warned when there was unequal pressure in the front and rear systems, but that it didn't transfer fluid between the front and rear systems. It seems to me that somehow fluid got transfered between the front and rear systems, as there are no leaks in the front system, but the reservoir went dry (I know the front brake reservoir had fluid in it because I helped him change front brake pads a while back). Can any shed some light on this for me?

Re: Brake Proportioning valve Question
 

the front resevoir is much larger cause the volume of fluid used by the front calipers is much greater than rear wheel cylinders...any chance this still has a drum brake master(both chambers equal size) cause it doesnt take much pad wear to empty the small resevoir

Re: Brake Proportioning valve Question
 

Its a disc-disc setup with a disc-disc prop valve, and a 3/4 ton master cylinder which has larger,equal size front and rear reservoirs. The fluid level drop is definitely due to leakage and not just pad wear.

Re: Brake Proportioning valve Question
 

If it is a 3/4 ton truck, the front reservoir is for the rear brakes. Opposite from 1/2 ton trucks.

Re: Brake Proportioning valve Question
 

Yes, I am aware of that.

Re: Brake Proportioning valve Question
 

I have never heard of the front bowl of the master cylinder being for the rear brakes on any GM car or Truck.

Please show us where you found that information.

Re: Brake Proportioning valve Question
 

Pics in link
http://67-72chevytrucks.com/vboard/s...d.php?t=666291

Re: Brake Proportioning valve Question
 

Not trying to make this into a 3/4 ton master cylinder discussion. But yes, the front reservoir on 3/4 ton master cylinders is for the rear brakes, and the front reservoir on a 1/2 ton reservoir is for the front brakes. Do a search for more info. In-Line Tube is a company that sells the hard tubing kits to do the swap.

Re: Brake Proportioning valve Question
 

I used to be a brake engineer for GM Truck; we do it all the time.

Normally it's due to packaging constraints, putting the smaller rear brake reservoir in front for clearance to a sloping hood.

See Astro/Safari van. My 74 Chevelle is that way, too.

Also - some designs prefer the primary piston in the master cylinder to engage first, which places it at the rear of the master cylinder. This primary system feeds the front brakes since most of the braking is done with the front.

K

Re: Brake Proportioning valve Question
 

Well you learn something everyday.

Re: Brake Proportioning valve Question
 

Attachment 1428456 bad second circut front seal, 1st circut gravity bleeds out 2nd circut. That's my guess. I would throw in a new master cylinder.

Re: Brake Proportioning valve Question
 

so did yo rebuild or replace the brake cylinder? did that take care of the brake issue?

Re: Brake Proportioning valve Question
 

I replaced the caliper/cylinder, and am monitoring the fluid levels in the master cylinder. I still can't figure out how the reservoir that fed the leaking cylinder was fully, while the reservoir for the non-leaking front brake system was empty. There must have been some kind of transfer or leak between the front and rear systems within the prop valve or master cylinder.

Re: Brake Proportioning valve Question
 

I will probably end up doing this just for the peace of mind. Master cylinders are cheap.

Re: Brake Proportioning valve Question
 

Hi all, curious if you figured out the fluid level / brake problem?
I have a '71 3/4 ton gmc and had a similar empty reservoir problem but could not find any leaks out side the cab anyway, just happened to see a stain on my new shoes and brand new carpet seems that the trailer brake switch under the steering wheel had broke inside and it kept draining the front reservoir 1 drip at a time. Funny I was looking out by the brakes for a leak.

Re: Brake Proportioning valve Question
 

In case the OP checks back.

The reason they use a larger reservoir for disc brakes is because there is no adjustment to allow for disc pad wear.

Drum brakes have automatic adjusters or on older vehicles you are required to periodically, manually adjust the brake shoes for wear.

On disc brakes the fluid slowly transfers to the caliper piston cavity and stays there. Disc/disc Master Cylinders can still have larger front brake reservoirs, because the front brakes, drum or disc, do most of the work.

Most modern cars have float switches in the reservoir(s). If designed right, the indicator light for "low fluid level" will light just before it is time for the pad scraper arms to start screeching on the rotors.
When that "low level" light comes on, you can check for leaks, but in most cases the pads are worn out. If you are in the habit of filling the reservoir on a car with the float switch, you will have defeated the early warning feature and you will have to remove some fluid before replacing the pads.

My question to the OP, did you check the front pads for wear?

Re: Brake Proportioning valve Question
 

The reason we use a larger reservoir for disc brakes is because they require a larger volume of fluid to operate the system, due to the large diameter of the caliper piston (as compared to the relatively small drum wheel cylinder diameter).

The fact that it accomodates pad wear is a secondary or tertiary consideration (...if at all. I've never heard it mentioned as a design constraint).

K