I have an R55 that I own, maintain, but seldom drive. Originally my car that my wife stole from me about four years ago (upgrade from the R52 she was driving) and then Daughter No.2 somehow managed to get her hands on it, and has been using it as a commuter ever since. Still in my household, still part of my fleet of vehicles (I still consider it MY car), and I still maintain it.

Daughter informed me recently, that the brakes suddenly and [seemingly] randomly "Go away" periodically. Alarmed, I had a closer look. I found it to be low on brake fluid. I'm guessing I put 2 or 3 ounces in it to top it off. I took it for a test drive. The pedal felt unusually stiff to me. Not what I expected, but it's been a while since I've driven it and I questioned, "It it just me?". It has good stopping power, most of the time, but every once in a while.... I felt a significant brake fade. During that test drive, I never could figure out why and I was having trouble replicating it.

I pulled the wheels and bled the brakes this morning. I got no air out of them, or at least none that was noticeable. Took it for a test drive again. Brake pedal still felt unusually firm. I can stomp on the brakes once... twice... and usually three times. The third starts to feel ever so slightly less powerful. ...on the FOURTH attempt, they fade bad. Still have brakes, but it no longer feels like "Power-Assisted" brakes. Like it's just you, and old school muscle. The same applies if you're not stomping on them, but just braking like normal. One, two, threeeee..... and done. Significantly drops off on the fourth. Light braking is the same, but the brake fade doesn't come until five or six pumps.

It only takes two or three seconds for this cycle to reset and repeat (meaning if you're not braking for a couple seconds, they come right back). It's a WEIRD scenario and not one that would come up that often, unless you're in stop and go traffic. ....which my daughter is often in stop-n-go traffic (gah!).

Is this a master cylinder failing? If so... I would expect to have a very soft pedal and braking power dropping off significantly all the time.

 

Is this 1, 2, 3, no brakes in quick succesion?

I would look into the brake booster. Or possibly the vacuum pump for the booster.

 

Yes. Relatively quick. If I slow it down, I might get 5 pumps before the power fades away.

 

vacuum leak at booster.
or vacuum m pump for booster failing.

 

I have vacuum at my pump. The line going from the pump to the booster, doesn't seem right to me. Like it has too much play/slop, on the booster end. But I can't see where it attaches to the booster exactly, as the line seems to pass through my firewall. I'm not sure how to get to it to check it out.

I've looked online, hoping to find a YouTube video or a parts diagram or something to help point me the way, but everything want to show my how to rebuilt/replace the vacuum pump.

 

Vacuum is relative, test the pump first.
https://www.pelicanparts.com/techart...mp_Testing.htm

Vacuum line goes in booster just below the master cylinder. Towards the inside.

https://m.youtube.com/watch?v=Ow-8l5Ry_3k