Leaking Master Cylinder Cup Seals

Today's master cylinders have two pistons that work in tandem to apply pressure to the braking system. These pistons have cup seals to seal fluid in the pressure chamber. When the brakes are at rest, the lip/cup seals rest between the vent and replenishing ports.

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When the brake pedal is depressed, the forward seal passes the port, creating hydraulic pressure that moves the pistons in the brake wheel cylinders and brake calipers. Unfortunately, when these seals wear, or contaminated fluid causes them to become soft and mushy, the master cylinder leaks internally. As a result, the brake pedal will fade to the floor as fluid pressure bypasses the seal. This pressure loss in the brake system can cause the vehicle to creep forward in gear.

Master Cylinder Leak Test

The system may have an external leak. Check the master cylinder's reservoir, but remember it may be low from brake pad wear or a hydraulic fluid leak in the system. External leaks appear as wet spots on the brake lines, fittings, and calipers. A small external leak behind the master cylinder can leak into the brake booster. Unbolt and look behind the master cylinder for a leaking seal.

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Check the internal seals by blocking the ports to the brake lines with pressure-resistant plugs. It's simple; if the brake pedal fades to the floor with the ports plugged off, the master cylinder cup seals are leaking. Conversely, a firm pedal indicates that the master cylinder is not leaking and that there is a leak or problem somewhere else in the system.

Master Cylinder Repair

Master cylinders are either replaced or repaired with a master cylinder repair kit. Check with the manufacturer before proceeding; honing can remove special coatings, resulting in premature internal leakage. It may be safer to replace an aluminum master cylinder with a new or remanufactured one.

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OK....now it is time to learn about brakes. My front brake is now doing the deal where it works fine but with steady pressure on the lever, it will pull all the way into the grip. No fluid leaking out of the system. Endless hours bleeding and re-bleeding until I think I have all the air out. I have read the other posts about this and one answer was that it MIGHT be the primary cup in the master cylinder. My question is, what eles should I check before tearing into my master cylinder? If the answere is nothing else to check then in I go. Can't wait! Commando 75,

If I read u right, the lever keeps pulling in toward the grip? That is a damaged cup seal, leaking internally. So it will not show up as a loss of fluid. If you release and re-apply you will get another pul, that will slowly subside.
When you pull the MC apaetlook for a small nick in the cup, maybe in the shape of the small breather hole back into the reservoir. This indicates the piston cups are not correctly orientate to the breather hole porting. I know because I made my own smaller diameter piston, and it worked fine until I had your problem. Also from my experience in making my own pistons, if the relationship with the piston to MC is not correct, I.E. when you pull the lever it does not fully stroke the cyclinder bore, or short strokes, it wil make bleeding a pain and you will end up with a spongy lever. I added a button to my piston, between the piston & lever and it bleed up like magic.

Good luck

Richard Once again thanks so very much for all the info. I took the MC apart today just to get it over with. As usual...nothing scary in there. Richard thanks for the ID of the cup seal as the problem. It looked OK to me but I will tear it apart tomorow and really check it over. Also Richard what do you mean 'added a button'? A spacer of some kind?

I can see where the brake hoses could cause the brake to feel odd but I am pretty sure the symptom of the lever pulling all the way in is a master cylimder issue. It feels exactly like under the constant pressure fluid is escaping to the other side of a seal. Commando, I am runnign a 1/2" bore already, are u running std 5/8"?
I made up a new piston to some dimendions that were posted by a local Norotn Commando Owners club. And I made the piston from alloy with a bronze end button, for the lever to strike on. With the NO dimensions in reference to lenght and where the seals are placed, I tried bleeding up, but could not get a full firm bleed. So I popped the piston out and made a 3mm longer bronze button, these are pressed into the end of the alloy piston assy, interfernece fit. And lo and behold after pushing the piston back in the damn thing had excellent feel withot even re-bleeding.
So if you have an aftermarket piston set up, it could have the same issues my piston had. But if the seal has a mark in it like a little round hole, that indicates it is too far out of the cylinder when pressure is building, and that causes the seal edge to get extruded out through the reservoir hole,

Hope this helps,

Richard