I am preparing to replace my master cylinder. I have the replacement unit and three liters of ATE DOT 4 brake fluid. How do I bench bleed the replacement master cylinder? Also any tips I should know about with this job? I intend to bleed the entire system but the process seems to be a major PIA. Is there a simple way to do it?

I got a rebuilt MC from autozone ,it included instructions on how to bench bleed saying that it's difficult to get all the air out with normal bleeding. bench bleed was the easiest part of the job, basically, holding the MC in a vice (or however you want to secure it... attaching two hoses from the outlets back up to the resevoire and compressing and releasing the rod several times untill there are no more airbubbles and you can hear and feel that it's pumping pure fluid (no bubbles) I then mounted the new MC and attached the brakelines by quickly and deftly slipping off the bleed hoses and screwing in the lines.. and small amount of air will be bled when you do the normal bleeding. then i did a full bleed on all 4 brakes and tada! they work great! :-)
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1991 white 240 auto

Like Niel, I did not bleed my MC. I left the plugs in the two fittings and filled up the MC with fluid. Then I removed one plug at a time, and waited for it to weep fluid (about a min or two.) Then, I simply connected the brake lines to the MC. Air bubble intrusion was minimal. I bled the brakes and got a nice, firm pedal on the first go-round!
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Speed Racer, '83 240 R, '74 164 E, '93 940 OL1 (Manchester, CT)

I've made my own pressure bleeder when I thought the rediculous price was too high. Used a piece of fuel line I had left over, and bought a one way valve from a pet shop. Any one way valve will do, including those in the vacuum system of the engine. Connect it to a nipple, open the nipple, pump away until no more bubbles, close nipple and move on to next one. Was about 20% of the price of the proper kit at the car spares shop, and worked just as well. I will never again revert to the old 2-person method where one pumps and the other open & close the nipple.

As for first bleeding the MC, not needed. You will get air in there anyway when you hook up the plumbing. I've removed a number of MCs (or rather 2 a number of times), and just fill them up after fitting. Fump a few times with lots of fluid in to fill the pistons, then you're ready to do the system bleed. You want to get rid of all the old fluid in the system anyway, and this ensures you will bleed until everything is out.

I am sort of confused. On one hand I am being told I need to get the MC bled using a closed circuit of plumbing...that is going to cost me more money to get that stuff and more time that I really do not want to spend. On the other hand I am being told to just install the new MC, fill it with fluid and pump away until it comes out clear...is that correct? The replacement MC I got from FCP has no plugs over the fittings...just open female threaded ports. I really really apreciate the help and would like to get this job done this weekend but really am running out of cash to buy more things to get the MC installed. Can someone confirm or deny that I can just install the new MC and pump it bled without special hoses?

There are kits available to facilitate bleeding the MC. Check with the supplier of MC to see if they have the kits. I usually mount the MC in place and attach the proper fittings to the outlet ports. A hose is then placed on the nipple of each fitting and routed back to the resevoir which is then filled with brake fluid. Using short strokes on the brake pedal you pump (circulate) the brake fluid in a loop and any air inside is expelled into the resevoir and escapes. It is imperative that the hose remain below the surface of the fluid so no air is drawn back into the MC through the hose. After all the air has been expelled from the MC you remove the kit fittings and attach the brake lines. You then bleed the system in the correct sequence as described in Bentley's (or a similiar reference)

Randy

Thanks for the answer. Is there any way I can use easily and cheaply purchased hose line from the hardware store for this, rather than buy a kit? What size hose would work? Also, I am having problems understanding why I need to bleed the master cylinder and then bleed the brake lines. Won't bleeding the brake lines also expel any air in the master cylinder as well?

Assuming (what a way to start) you can come up with lines leading back up to the reservoir (try places like NAPA , Autozone, Pep Boys, etc.), you can do it in a vise. Same procedure but use a blunt wooden dowel or what have you (and muscle) to actuate the piston rod.

When you're satisfied the air is all out, leave the lines in place and install the MC on the booster. Next, find something to wedge on the brake pedal, depressing it about an inch. This will prevent loss of fluid from the ports when you replace the bleed lines with the regular hard lines. (Don't ask me how this works but I've done it for years, any time I had to open a brake line.)

With hard lines hooked up, remove the pedal block and you're ready to bleed. If you can, have someone press down on the pedal and open the MC fittings slightly (one at a time) for a brief second or two, using a rag to soak up the fluid. This is to expell any air that might have got in as the lines were exchanged.

I think the reason for bench bleeding, and what I just described, is because it's hard to force air bubbles down and out from the highest point in the system. Hope this helps,
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Bruce Young,
940-NA (current)
'80 GLE V8 (Now gone)
'83 Turbo 245
'73 142 (98K)
'71 144 (track modified)
New 144 from '67 to '78
Used '62 122 from '63 to '67

You need something to screw into the ports on the master cylinder to attach the hose to. If you had access to a parts car you could clip the lines that attach to the master cylinder and unscrew the fittings. You could then attach the fittings to the new master cylinder and bend the old steel brake line to place the ends in the resevoir.

Can't give you an answer to the question of why it is necessary to bleed the master cylinder other than it is to expel the air and insure you remove all of it before attaching the lines. Any air that would be trapped in the master cylinder would make bleeding the lines a mute point.

Make sure you wash any brake fluid off of painted surfaces as it can damage the finish.

Randy

The MC on my 740 turbo has its own bleeder screw. It is likely the bleeding procedure for this unit is merely one of installing the MC in place, attatching the hard lines (two in my case) then opening the bleeder and pumping out the air that way?
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"Can't understand why people abort Volvos, either"

I ran across this for the first time recently on an 88 turbo sedan. I had never seen it before. I have never seen a replacement MC with it.

Randy

The challange is, how do you use the bleeder screw to bleed the m/c?

I couldn't get a hose on it to drain off the bled fluid. the valve is nearly wedged tight against a useless flange of sheet metal welded to the shock tower.

All I could stuff a paper towel between theer and squirt out some fluid. this was after I re-routed fluid back into the bottle from some spare lines I put onto the hard line outlet ports.

I am still not happy with the pedal feel, despite three bleedings.

Too much initial travel before engagement.
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"Can't understand why people abort Volvos, either"