Bench bleed trouble


#1

Finally got around to bleeding the brake master cylinder, but found my old thread about bench bleeding was closed, so:

Ran into trouble bleeding the brake cylinder. I only have one fitting for the port, need two more. However, I have some plugs in my vacuum pump kit.

Can the cylinder get bled with two ports plugged, one port with the fitting, then rotate the fitting and plugs to each of the other ports?

This could save me a week and $20.


#2

Put the master cylinder in a vice and fill it with brake fluid. Plug the outlets with your fingers on one hand and with a screw driver press the piston down until the air is blown out of all the ports and liquid pressure can be felt. Install the master cylinder and attach the lines, top off the fluid and with one finger depress the brake pedal about i inch repeatedly until back pressure is felt then press the pedal firmly. It should be fully bled.


#3

Thanks for the concise guidance @"Rod Knox"‌ - I’m almost there…


#4

Finally did this. Some remarks: The bubbles can be very small, and flow very slow. I had to put the tubes near the fluid surface, and time the piston pushes so they didn’t get sucked back into the cylinder. I angled the cylinder up and then back down to help clear bubbles. I also let the system rest and then resumed.


#5

My experience is limited to motorcycle master cylinders and so this might be useless info, but I have found that lightly tapping on any narrow areas can encourage the tiny bubbles to move along and up into the reservoir. Also rather than simply pushing on the piston try putting a little pressure on it and then wiggling the pressure so the fluid is not just pushing the air bubbles around, but is shaking them a bit as well. It seems to keep them moving, instead of sticking to the inner walls of the lines and passages.

Sorry if that sounds screwy. It’s hard to explain. The bubbles are tiny and they move at their own pace, and I was trying to hurry them up a little.


#6

I got away with one on my 87 Mazda truck. I jacked the car up until the lines were almost level, cracked the b-nut and had a helper press the pedal to bleed it. The brakes worked great. Sometimes luck works. Sometimes not.


#7

ok last one I’ll follow up on, in case this is a Foie Pas.

in this case, I had to assemble one brake line to the cylinder in order to install it. I had to install it to discover this. Anyways - long story short : after some combined gravity bleeding and gentle air-pressure-on-top-of-the-brake-fluid bleeding, it worked. solid brake pedal, brakes work, etc.

… I mean, one of the worst things about searching online forums is finding a good topic but then it just stops before any conclusion is met… oh, nice website redesign BTW.


#8

You did well there. The brake lines aren’t high enough for great bleeding so I jacked up the left side of the 87 Mazda truck and bled them well. If they were high enough there would be no need to bench bleed. Oh well.