Bench bleeding brake master cylinder w/separate reservoir

WesternRoadtripper1
Sat, 05 Jun 2021 22:06:03 +0000
Maintenance/Repairs

The pedal has been questionable the past few months, but I’ve been driving so rarely, I hardly noticed it. This week it became an obvious problem; it’s time to replace my master cylinder. There’s no leakage at any wheel, the reservoir is NOT low, and front calipers are less than a year old. The rears were bled by my mechanic last fall after I had replaced the front calipers and couldn’t open one of the rear bleeders to fully purge all the old fluid.

Driving hasn’t presented a problem yet, even on the few trips this past week pulling my small utility trailer. It’s only when I hold the pedal down like at a red light, that my foot slowly sinks toward the floor. If I pump it just once, it’s ok, but still sinks a bit.

I’ve found videos for bench bleeding a master cylinder with the reservoir built into the unit, that seems logical enough. But the Sienna has its reservoir situated several inches away, connected to the cylinder via hoses. Because of that, I’d be guessing about how to bleed it properly. Guessing about brakes is not a good practice IMHO.

The m/c itself sits back under the cowl area, which appears to be why the reservoir is separate. There’s no space under the m/c to capture brake fluid being squeezed out, and in this case, one brake line – the one toward the front of the car – attaches at the top, not the side. So I don’t see how to bleed it once installed. At the very least, it would be a mess. Or is that top mounted brake line the solution to how to bleed this - air escaping at the top? Hmmmm.

If I don’t get it done this weekend, I’ll be waiting at my mechanic’s door Monday morning, but it’s doubtful he would be able to jump on this right away.

It would really help if I could replace the m/c this weekend. It’s bad timing: I have a road trip planned starting Monday, meeting friends for camping and kayaking, and I’m not comfortable going with sketchy brakes.

I’d be grateful for anyone who could explain a proper way to bench bleed this master cylinder. Thanks in advance.

3 posts - 2 participants