Not again!!

[ncgirl05]

I have a previous thread on this issue: http://www.peachparts.com/shopforum/sl-forum/325268-brake-issue-77-450sl.html

Today driving home from work my brake pedal once again lost pressure, and went to the floorboard with just enough "umph" left to stop it. Pumping the brake several times brought it back to normal, but had to do this several times to get it to stay this way.

I see in the previous responses that air in the system could be a possible culprit - would the above scenario be possible due to air in the system? I soooo don't want to go through the whole master cylinder replacement again....

And are there any other reasons? This has now happened with the old master cylinder and the new master cylinder.. all other parts are okay to the best of my knowledge, fluid is full, etc.

And also - how would I know if it's the brake booster?

[meltedpanda]

sudden loss of pedal , with out any tell tale leaks on the external lines, most likely points to master cylinder, or air intake.
Inspect the lines closely at the calipers, and along the bottom of the car and in the engine bay, if you find a leak , then air could get in there , also look closely at calipers for leaks

[ncgirl05]

I just discovered I should have been using dot 4 fluid instead of dot 3. I am now wondering if that has been the real problem because I only had the loss of brake happen when the car was hot, never cold. Thoughts?

[RadioTek]

That shouldn't make any difference, unless they were mixed. Even then it really shouldn't have any problems, theoretically. It sounds like an issue with the master cylinder, the fluid is leaking past the piston, or there is air in the master cylinder. Could it possibly be that it wasn't bled properly??? On my old truck, I had to 'bench bleed' the master cylinder when I replaced it, but when I did the SL, I didn't. But then I had a pressure bleeder by then.

Just a guess,
Scott