Brake problems

I have a 1985Chevy 2500. after sitting for about 3 or 4 months the rearmost brake fluid reservoir is empty. Does the rear reservoir supply fluid for the front or rear brakes?

That’s the primary reservoir so it supplies brake fluid to the front brakes.

If you’re losing fluid from the reservoir, remove the master cylinder from the brake booster without disconnecting the brake lines to see if the master cylinder is leaking into the brake booster.


This being the smallest is still the primary?

I don’t know whether the fluid loss is actually from the reservoir or from a line or a wheel. this is why I am wanting to know which brakes this reservoir is for. I am going to look at the wheels and all the lines for that set of brakes.

I have not checked on the leak yet but I was wanting to know if fluid has leaked into the booster, will it damage the booster? I have always heard that the smaller reservoir is for the rear brakes, are you sure this small back one is for the front?


Yes . . . the brake fluid can damage the diaphragm of a vacuum brake booster

Do you even have a vacuum brake booster?

I’m asking, because many GM trucks and vans have hydraulic brake boosters

if its the smaller section, wouldn t that go to the back wheels?

The reservoir closest to the booster is the primary feeding the front brakes. Your master cylinder looks like this, right? I wouldn’t necessarily call that one smaller.

No, my reservoir is all metal. It is for a 1985 Chevy. The back reservoir on mine is definitely smaller.

does a back tire have a stain? maybe one of the wheel cylinders gave out

Both back wheel cylinders were replaced only a little over a year ago. I see no sign of a leak on any of the wheels but I don’t know how long the reservoir has been empty, the truck has been sitting for at least four months now.

You must have this master cylinder.
The primary is much smaller, but I cannot see swapping the primary and secondary around. The piston closest to the booster moves first when the brakes are applied, then the secondary moves after pressure builds up in the primary. Maybe being a heavy duty truck means a bigger reservoir for biggr rear brakes.

Yes that looks like it.

What would cause three master cylinders to go out within three years? The truck is driven only about six months out of the year and then only short local trips. I am tired of replacing master cylinders. They all seem to go out causing the rear reservoir to leak into the power brake booster. I have had to change this once as a result of this.

Are you buying new master cylinders or rebuilt? Usually when something like this occurs, something else is the actual root cause.

If you are using rebuilt (remanufactured) master cylinders, the root cause could be with the company or factory where they are remanufacturing the master cylinders. Your master cylinder is an unusual configuration where the larger reservoir is on the front instead of the rear and the rebuild facility may be using a generic plunger that is made for the larger reservoir to be at the rear.

It may also be that you are using the wrong master cylinder. You are replacing the old one with a new one that matches the old one, but are you absolutely sure the old one was the correct one in the first place? You may need to go to a dealer and see if they can match the correct MC to the VIN.