Leaky brake

I have a '99 Silverado 250 with a leak between the brake reservoir and the master cyl Mechanics tell me there is no gasket How can I get it to stop leaking?

You need a new reservoir. It comes with a new seal and lid. About $12 at Autozone.

Missileman, I believe the seal through which the fluid is seepingto be a part of the Master Cylinder rather than the Booster pushrod. There’s no gasket between the two, but generally there’s an “O” ring with a retainer ring (snapring) in the MC that keeps the fluid in the system.

@mountainbike…I was just using the OP’s description to narrow down the problem. The master cylinder does have an “O” ring and snap ring on the rear and it’s also a common source of leaking. I believe the OP is talking about the cheap plastic reservoir on top of the master cylinder. Maybe we can get some clarification from @oildog.

Here’s what the master cylinder reservoir looks like.

Too bad the O-rings for the reservoir don’t come with the reservoir. The O-rings are part of the master cylinder.


Missileman, I had a brain fart. My eyes saw “reservoir” and my mind saw “booster”. You are, of course, correct. Humble apologies.

The O-rings should be easily replacable. They don’t experience pressure, so there’s no special hardware, just removal of the reservoir and replacement with some generic O rings.

The leak is a 2 bolt connection between the hydraulic booster and the master cylinder .It is a metal to metal connection with no gasket between. But can I put a liquid type silicone gasket material between the two?

Oil dog, I need to be sure we have our terms correct. The reservoir is the white partially see-through tank that sits on top of the master cylinder, and the booster is the metal canister that’s between the brake pedal lever and the master cylinder.

If the leak is between the small plastic tank and the master cylinder, it’s a simple O-ring replacement. If the leak is between the booster can and the master cylinder, I’d recommend a new master cylinder. If that O-ring is leaking, the ones around the pistons may be getting old too, and their loss would mean loss of braking. Besides, that O-ring prevents brake fluid from weeping out when you let up on the brakes, and loss of fluid could manifest itself as a loss of brakes.

The booster that the Silverado 2500 came with is vacuum operated, not hydraulic. There are hydraulic brake boosters available, for use with “boosted” induction systems (they don’te develop vacuum like naturally aspirated engines do), however unless the truck has been modified you should have a vacuum booster.

Can you clarify where the leak is? Did my dumb mistake and mention of the booster confuse the issue for you?

Diesel trucks usually have a hydraulic brake booster. Is it leaking brake fluid or power steering fluid?

Good point Nevada,. Did they offer the '99 Silverado 2500 with a diesel?

Some GM trucks with gas engines have hydroboost brake assist systems. Here’s the hydroboost unit for you truck. http://www.rockauto.com/catalog/moreinfo.php?pk=1571370&cc=1353697

If the leak is between the master cylinder and hydroboost unit the master cylinder rear seal is leaking and the master cylinder requires replacement.


Cool. I learn something new every day.
So my original bonehead error apparently put me on the right track. Go figure.