There's no gasket, a leaking master cylinder means it needs to be replaced. It's about an hour's worth of labor at the shop plus the part's cost.
The process of bleeding brakes: The shops have a power bleeder machine which allows one man to do the job by himself. For us backyard mechanics, the operation will require two people. It is not a long process, but may require removing each wheel to get access to the bleeder fittings. One person operates the brake pedal, a second person opens and closes a bleeder fitting on each wheel. Starting at the wheel furthest away and working forward, to each wheel:
1. The brake pedal is depressed. DO NOT LET UP on the pedal until the bleeder man says to do so. The pedal can only be let up when the wheel bleeder screw is closed. You want to force the fluid to go in one direction -toward the wheel.
2. The bleeder fitting is opened. The brake pedal sinks to the floor as new brake fluid and any air pocket is pumped thru that line. Brake fluid exits thru the bleeder. An air pocket exiting the line will sound like it is spitting. When you see new brake fluid coming out, the entire line has been flushed.
3. The bleeder fitting is closed. This prevents air from being sucked back into the wheel cylinder when the brake pedal is let up.
4. The pedal operator is then told he can let off the brake pedal.
5. Check the resevoir in the master cylinder every few bleeds. Do not let the fluid go low as it will just introduce a new air pocket.
6. Repeat process to each wheel.
NOTES: When replacing the master cylinder, ALWAYS us a flare wrench removing the brake lines. NEVER attempt to do so with open ended wrenches. A flare wrench is a special boxed end wrench with a notch cut out of the side so that it can get over the line and fit fully on the brake nut fitting. You will strip the fitting if using an open ended wrench as they are on very tight.
There is a trick to replacing the master cylinder without bleeding each wheel which I have successfull done twice. I've also only done this with salvage yard master cylinders which are for the most part purged of air. Never tried it with a new or rebuilt master cylinder.
To explain: After removing the master cylinder, the air pockets you have introduced will be at the ends of the four brake lines you've removed and in the new master cylinder. This is the air that needs to be purged. Air rises, fluid sinks. If the shape of the lines has a bend that will trap air, this may not work.
Procedure:
1. Reconnect the lines but do not tighten - leave them loose so air can escape.
2. Introduce vibrations to the lines and master cylinder by tapping against them with a wrench. Even better if you have a virating device available.
3. Using your hand, wiggle the brake pedal in-out ever so slightly - no big pushes. This forces small amounts of brake fluid into the line to replace the air. Air will rise and hopefully exit thru the untightened fitting or purged thru the master cylinder.
4. Repeat the tapping and pedal pushing steps for up to five-ten minutes. Take your time on this as it only works if you get all the air. You may notice bubbles in the master cylinder as you do this. This is air being purged, a good sign.
5. Tighten your line fittings and test the brakes. If the procedure didn't work, the brakes will be spongy (i.e. low pedal on first depression and a need to pump them once means air in the system). If that's the case, then the wheel bleeding procedure will have to be done.