Draining master cylinder

[mc1984ss]

I had to pull the proportioning valve / combination valve out of a car. ( Everything was nicely painted) Needless to say this was a messy job. I know I will never get it back in without making a huge mess and brake fluid running all over the place. If I allow the master cylinder to drain but don't press the pedal will I need to bench bleed it again? I think I did this years ago and since I didn't touch the pedal I was fine. Thanks in advance I appreciate it

[79 Camaro]

I was thinking that you can find flare fitting plugs that you can screw into the master to prevent it from draining. Maybe as close as NAPA? Or another thought is buy the correct master flare fittings along with a couple lengths of brake line and aim the lines straight up.

[Tsaints1115]

And line the engine compartment with painters plastic.

[mc1984ss]

I have caps on the brake lines right now. My concern is when I go to reinstall the combination valve. When I pull the caps off thee will be fluid running all over the place. I would really like to avoid this. I'm wondering if I let the master drain but don't press the pedal if I will have to rebleed the master cylinder.

[Tim C]

Yes you'll have to rebleed. The fluid in the actual bore will still leak out even after draining the reservoir. There's a port between the bore and reservoir that is open with the piston all the way back (pedal released) so the bore can refill or replenish as the pads wear. With it open the fluid will run right out the line port.

You can push the piston in a 1/4 to 3/8 inch and it'll cover that port, blocking fluid from leaving the reservoir, it'll also cause the fluid to leak from the ports much slower due to the vacuum build up with the refill port blocked, similar to the glug when you pour oil from a bottle tipped upside down.

If you could rig a way to hold the piston til you get the lines on that'll help. It's also a great way to keep from emptying the master while you change a caliper or something, which is why I brought it up. At work I'll jam an old broom stick between the pedal and seat so it won't run the master dry while I change a caliper or brake hose.

Another thing I do is to lay plastic trash bag under the master, and then put old rags down to soak up as much fluid as possible.

If the paint is regular base/clear, the fluid won't hurt it as long as you wash it off right away. If it's enamel or lacquer paint it may stain.

[Rick Dorion]

I've been able to do it quickly with minimal spillage. Used lots of blue paper towels underneath.