I have an 85 gsl and I believe my brake booster is going bad. Every time I shut off the car, I hear air hissing from the area of the booster and MC. Can’t find any new replacements, curious if I can use the one from Mazdas glc from the same year. I saw it’s in stock on rock auto. Also read the rebuild instructions in my fsm, but i don’t know where to get new seals. If anyone can help, it would be greatly appreciated. I don’t want to lose my power brakes.

 

I'm assuming your foot is off the pedal when it hisses?

There's also a check valve. It's easy to remove the hose and see if air only goes one way.
If you have a vacuum pump you can also check if the booster holds vacuum, just to be sure.

Otherwise I have the booster off my '82 that was in use until June last year when I did my brake upgrade. It's a Cardone reman that I installed about 10 years ago.
You can have it for the cost of shipping.

 

I had mine rebuilt for like 80$ at a local shop.

 

yes, foot is off the pedal when I shut off the car and it hisses. But There’s also a leak somewhere in the fluid system, I believe it’s on top of the master where the reservoir sits. there is air in the system so I think that is affecting my diagnostic on the booster. Got a new master but I don’t want to replace it and bleed the system all to find out my booster actually is bad and I have to remove it all and do the process over again. I appreciate the offer on the booster, just might take you up on it. I’ll know if I need it or not on Saturday.

 

There are some nice used units out there and j_sto has you covered! Just make sure it's the same as I think there were some different boosters depending on year/model.

 

Often times the piston seal on the brake master cylinder fails, and when this happens, brake fluid seeps out in between the master cylinder and booster. When this happens and brake fluid is left on the booster, the paint on the booster wrinkles and the moisture in the brake fluid will cause the booster to rust where the brake fluid has settled on the booster. Over time, the rust will make the metal on the booster so thin and pitted that an air leak will form. Once that happens, the booster body must be replaced. It can be repaired by welding in new metal, but it may be a better option to repalce it. With that said, I would verify that the booster body is good before looking to rebuild it.

 

This was what killed my booster, the seal leaked and made my booster into a coconut.

 

Get a T3 booster delete. Nothing to worry about after that...

 

Except you have to brake hard as hell to stop .

 

It's not THAT tough. It'll make ya grow some hair on your chest. P:

 

NA Miata brake boosters work and have various assist vacuum ratios depending on the year. You may need a manual proportioning valve instead of the stock Miata one to get the balance right.

 

yes I’m aware of this, read some threads that stated this problem, tomorrow I’ll get to check out the car and figure out what’s going on. Hoping my master Isn’t leaking internally.

 

You would be able to visually see the bubbled paint or rust on the booster from the fluid that leaked out from the master cylinder if that is an issue on your car. It should be a fairly quick diagnostic step and if you need any help in determining if the master is leaking, feel free to post a picture and we can chime in and help you determine if it is leaking or has leaked in the past and wasn't cleaned properly, leading to the booster body rusting.

 

Replaced the master today, got so carried away with the install I forgot to take any pics or video or even inspect the inside of the booster. Remembered after I was done bleeding the brakes. I looked at the booster face and it was definitely grimy but no major paint bubbling, I think i caught it early. Cleaned it up and looks good now, brakes feel good, not mushy, but there’s barely any travel in the pedal. I read something about adjusting the rod on the booster so it meets the cylinder rod perfectly or it’ll feel like the booster or master is bad. Curious if that is the case. Again, didn’t think about checking that during the install. I’d really make a terrible shop mechanic lol.
anyhow, it brakes good, I don’t have to press very hard on the brakes so I believe the booster is working properly.