On my '79 the brake pedal is rock hard. I've been assuming that the booster is the problem but want to check here first to see if there is anything else I should check before diving into replacing it. Check valve maybe? From the research I've done it looks like it's hard to find a replacement...any suggestions?

Thanks,
Rich

 

you can take a vacuum hose off and test the booster. if its not that, i'm not sure it would be though

 

Thanks j9fd3s! I'm not sure what you're saying here...could you elaborate?

Thanks,
Rich

 

the booster should more or less hold vacuum.

 

bad booster OR bad booster hose I bought a used one while I had the original rebuilt by Rock auto/cardone for about $85

 

If you can't find a replacement through here or other places, an early NA miata brake master and booster fits the same bolt pattern. There's varying levels of vacuum assist depending on the year you buy. A miata.net post by Keith Tanner goes into all the different varieties. Once that's bolted in place, adjusting the factory bias valve or installing your own is the next hurdle. Overall it fits quite well. I've seen this done on an 83 and I've mocked it into my 84. I don't think the unique spacer from the s3 is needed, I can check if interested.

I think this is the post I read years back when looking into this.
https://forum.miata.net/vb/showthread.php?t=363284

Edit: This assumes that the S1/SA bolt pattern is the same as the S2/S3/FB

 

the pattern seems to be some SAE standard or something, Mazda from 1977 on, and Ford and Nissan, Subaru all are the same.

the FC guys like a Subaru master/booster setup as its big (over an inch in diameter)

 

had that rock hard brake pedal, put clamps on the hoses, FIXED !

 

I think we may have solved this problem, but want to see what others have to say. My brother came over to help me look at this and decided to test the check valve. When we went to remove the hoses at each end we found that many of the hose clamps were looses...we could twist the hose without loosening the clamps. After we had removed the check valve and tested it we reinstalled it and made sure all the clamps were tight. When I took it for a test drive I found that the pedal felt like it should, at least from what I can remember...it's been a while since I've driven this car. I'll drive it some more and see for sure.

I'm assuming a vacuum leak could cause the pedal to be hard...am I right?

I had been wondering what midnight mechanic was saying, but after finding this problem I'm thinking you didn't have clamps on the ends of the hoses and solved the problem by installing them, right? I'm the original owner and know for a fact that these clamps were on it from the factory

Thanks,
Rich

 

Yes, it sounds like you figured it out; the Brake Booster uses intake plenum vacuum to multiply the amount of force going to the Brake Master Cylinder Rod. The tubes which route this vacuum source must be sealed to allow the Brake Booster to work properly. Any air leakage will prevent the Booster from generating assisting force, and the pedal will feel hard, i.e., harder to stop the car than WITH boost helping. Additionally, that one way valve inside the vacuum source line to the Brake Booster is to capture a bit of vacuum inside the Booster in the event the engine dies and the vacuum source is lost. This way, you have 1 or 2 pedal presses before the vacuum is lost along with your boost assistance.

If you sealed the vacuum hose with clamps and that fixed the problem, then your hoses are probably dried up and cracking. Take a very close look at them and replace if necessary. Losing brake boost assist when you least expect it cohld lead to a crash.

 

I should have also said that I removed the brake booster hoses, and did a vac test with a vacuum pump. It wouldn't hold a vacuum.


Then got in the car, inserted key in the ignition...........I drove to HD with pliers. flat head screw driver and philips screw driver, bought the right sized clamps. Opened the hood in the parking lot, removed both ends of the vacuum hoses. Calculated the right compression to use on the pliers as I slid the clamps over the ends of the hoses

....gently lowered the hood, pressed it so it latched.................

this weeked, I'm gonna check those brake booster vacuum hoses for cracks, you got me worried.

 

Thanks LongDuck! These hoses are original, so I'm sure they are dried out and should be replaced. Since I am the original owner I hate to do that...I love originality. But I also love living lol. I think all hoses are pretty standard chunks of hose except the one at the passenger's side of the engine. It has a kink in it to account for engine flex. Do you have a suggestion for a place to get replacements?

While I'm at it I should probably replace the heater hoses...they are original as well LOL. I have replaced the big hoses...but just once.

Rich

 

If you are going for originality, I bet Mazda still has these parts in stock at reasonable prices too. I post my thread about the poc I use for ordering stuff. His name’s Ray and a great source for the rotary community. I have nothing but good things to say about him.

You can also get all those from rockauto for a little less money, but not by much.

Use the parts fiche found on foxed and send Ray an email with the list of Mazda part numbers to request a quote. $15 flat rate shipping for most orders. I’ve had to describe the parts I wanted a couple times because miata parts fiches are not as detailed as the fb ones.

Edit: his contact info should be buried in this thread.

https://www.rx7club.com/1st-gen-general-discussion-207/mazda-part-availability-1150700/

I can PM you his email and a list of the hoses you’re interested in if you aren’t used to using those catalogs.