Strange thing happened other day

[dcrolin]

Hello All...I had something really weird happen the other day and I thought maybe someone here has some insight... I own a 91 Rex Si MT w/77k miles, everything is stock...So anyway, the other day I decided to turn on the air (which I've only done once since I bought it 1.5 yrs ago) and drove it for about 30 highway miles...then when I turned off the A/C and went to brake, my brake pedal gradually sunk to the floor...luckily I only had a few more miles to home. I pumped the brakes to build vacuum, but it didn't help. the next day I got in it and the brakes work totally normal.....ANY IDEAS...Thanks for any input.

[Hybrid1990crx]

thats strange the a/c should not be related to the brakes at all. maybe just a coincidence that it happened then.

[jimmym86]

I've heard of this happening before and I'm not quite sure if I remember what causes it. But IIRC it can happen from driving (on a highway) for long periods of time. I don't think that the A/C has anything to do with it...(hopefully someone can jump in and explain this for us!)

I've had this happen to a few of my cars before...Hop on the interstate, drive for a few miles, get off at the exit and the break pedal goes to the floor, pump it a few times and it works as if nothing is wrong.

[johnb16a2]

I'd consider replacing the master cylinder, I agree the A/C thing was a coincidence.

[4G4D Store]

You could just have old brake fluid in it - or a air bubble in the line. Brake fluid boils at a low temperature, so on really hot days if your system isn't in good shape it can do that. I'd flush the whole system with new brake fluid and see if that takes car of it, then if it doesn't, I'd look at either replacing or rebuilding your master cylinder.