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Brake Master Cylinder Bleeding

ADVNTURR

I worry when rattles stop
Location
Cleveland, OH
I am in the process of upgrading the single diaphragm booster in my 91 XJ up to a dual diaphragm booster out of a 95 I pulled from a junkyard (I am swapping in the whole booster/master cylinder/proportioning valve assembly), but I am having a rough time trying to get the system to build any pressure. I managed to get everything bolted up and made some adapter lines to get from the location of the old proportioning valve to the position of the new one. I then started trying to vacuum bleed the system and after pulling ~1-2 reservoirs worth of fluid through the system I still had no pedal pressure.

I regrouped and did some more research and realized that I probably should have bench bled the master cylinder. Last night I pulled the master cylinder off of the booster and proportioning valve and set it up in my vice with some clear lines running back up into the reservoir. When I pump the cylinder I am getting good flow from the first port (closest to the booster/firewall) but the second port is not doing anything. If I disconnect the fitting from the second port fluid just drains out.

I do not have any experience with this style master cylinder, but I am assuming that both ports should be moving fluid when the cylinder is pumped? Does this sound like a clogged port on the cylinder, or could it be that I just got hosed by a junkyard part and need to rebuild/replace the master cylinder?

Any thoughts would be greatly appreciated
 
Both ports should move fluid about equally when neither is blocked. To get it bled really well, you want to bleed one at a time though (at least how I did it...)

Block off one port (a pair of vise grips or hose pinch forceps works great, make sure the rubber hose doesn't blow off the metal line from the pressure though :banghead: don't ask, that shit really burns when it sprays in your eyes BTW) and cycle the piston till no air comes out the other one. I find it helps if I squeeze the other one closed while releasing the piston, means the fluid only moves out the hose and therefore bubbles don't get sucked back down as you release it. Then once that port is bled, pinch that hose off and do the same procedure on the other line.

You may have better luck getting it to bleed fully if you rotate the master cylinder in the vice till the ports are at about a 45 degree angle facing up, worked for me, I think it got the air to move further toward the ports in the cylinder without pressing the piston in.
 
You can also get a good bleed by putting solid plugs in the outlet ports. It takes a little longer though. Either way will require a lot of changing angles in the vise, short jabs alternating with full strokes. You'll know you're starting to get somewhere when the fluid bubbles up above the inlet ports in the reservoir as the piston is going in.
 
kastein, the 45deg tip seems to have done the trick. I re-attempted the bench bleed last night and managed to get both ports pumping fluid. Thanks for the advice.
 
No problem. I figured that one out after bleeding it unsuccessfully once in the vehicle and once on the bench.

FYI... modern brake fluid does not seem to remove 2-stage automotive paint. I discovered this the third or fourth time the damn rubber hoses blew off the fittings from the pressure, around the same time I discovered that brake fluid stings when it gets in your eyes :looney:
 
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