Brake issues
 

Hi, I just picked up an 84 rx7. Im not sure which model it is because it has no options but it does have rear disks. Anyway, the brake pedal gets very soft when at slow speeds. It is fine at speeds above 15 mph but below that it acts as though there is air in the lines. I bleed the brakes without much success and cant find any apparent leaks. Any ideas? Thanks

Is it a carburated 12A or fuel injected 13B. If it's carbed then you have either a GSL which I don't think you do since they came with power windows and mirrors. It could be a GS or G model with the GSL rear end swapped in. Not sure about the brake problem. Could be your master cylinder has worn out seals inside and is allowing small amounts of air in.

most of the time it's air in the lines. bleed your breaks.
how are your break pads ?

The pads are good. I have already bleed the brakes pretty thoroughly. I am looking for a parts car so i will maybe try swapping out the master cylinder.

Kinda strange, because air in the lines would have the same effect whether you were going fast or slow.

And what exactly is the symptom that you refer to as being just like air in the lines?
Pedal gets all soft and squishey? Or does the pedal get harder when you're going slow?

When you mentioned the difference speed makes, it made me think of the check valve in the vacuum line. If that gets gummed up, you would have great brake feel at decent rpms, but once the rpms drop so does the vacuum and you end up with a rock hard pedal.

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The only part of the braking system which is affected by engine (and therefore vehicle) speed is the brake booster.

As a test, see if your pedal feel symptom varies at different engine speeds, with the car stopped and in neutral. If it does, your booster is hurting, or you have a leak or other problem in the vacuum line that feeds it.

Definitely sounds booster related with those symptoms. As suggested make sure theres no vacuum leaks ( but your idle would probably be unstable if there was ) and make sure that check valve is there and working correctly. Just blow thru it to check it. Its function is to hold the vacuum on the booster once it sees some vacuum.

the pedal gets very squishy when the engine is around idle. It doesn't happen while driving down the street until I slow to stop. I can pump it up to a normal pressure, but it goes squishy again within seconds.

Check/replace the vac line to the booster, it may have a tiny leak that only shows up during idle. If that doesn't fix the problem, have the brake booster checked.