Leak Between Master Cylinder and Brake Booster

[Cal2121]

I have a small leak between my brake master cylinder and brake booster. It sort of seeps out slowly. Is there a gasket between the two parts that I can replace to fix the leak, or do I need to do something else? I pulled up on the picture schematics but can't really tell. Thank you for any help.

[HIO Silver]

No gasket. In fact it should be dry. 'Sounds to me that the MC is leaking out its back side and needs to be replaced.

[Ranchero50]

Ditto. Mine does it too, 2nd replacement that leaked at the same spot. Planning on dealing with it for a year and hoping the rebuilder get better.

[ultraranger]

The rear primary MC piston seal is bad and letting brake fluid leak out the back end. The other bad part is brake fluid has probably leaked over inside the brake booster too. The brake fluid will eventually break down the rubber diaphragm inside the booster, causing it to fail.

Both the MC and the booster will need to be replaced.

[Cal2121]

Thank you for the information. I was worried that was it. Looks like I'll replace the MC and booster. Do you guys have a recommended brand? Not something I've done before.

[Mach428]

Since you will need to remove both in order to replace, why not remove them and use a pair of snap ring pliers to remove the plunger from the MC. It sounds like it needs new O-rings and if it works fine otherwise, this may be all that you need. Once removed, you can inspect the booster to see if there is much if any brake fluid in there. It could only cost you a few bucks.

[ultraranger]

There aren't any O-rings between the MC and the brake booster and no O-rings on the primary or secondary pistons inside the MC itself.

The fluid leak is a result of a bad MC bore and/or bad cup seals inside the MC. There are two pistons inside the MC --the primary piston for the front brakes (located at the rear of the MC) and the secondary piston for the rear brakes at the forward end of the MC.

Each piston has two cup seals, a primary seal at the forward end of each piston and a secondary seal at the rear of each piston. If brake fluid is leaking out the back of the MC casting, it means the secondary cup seal of the primary piston is bad.

A brand new (not rebuilt MC) can be had for $35-$50 dollars.

This is a diagram of the internal workings of an all-wheel drum Ford tandem MC. While a disc/drum MC will be slightly different, the piston cup seal configuration will be the same.

[Mach428]

I stand corrected. O-ring should have been "seal" as there has to be some sealing device (cup seal) to prevent brake fluid from leaking past the MC piston. My point is that it may very well be something quite simple compared to replacing everything.

Thanks for the detail