My master cylinder (manual brakes) has had a slow leak since I bought the car. I finally got around to purchasing a new (rebuilt) one, and despite following the bench bleeding instructions included could NOT get any piston resistance after several minutes, although no further bubbles were coming up through the fluid in the reservoirs. Finally I said "screw it" and installed it in the car, hoping that I could successfully bleed it 'in the car' during the process of a regular brake bleed.

Oddly, when I bench bled the old unit after removing it, within 6 or 8 strokes of the piston it was offering stiff resistance to about 1/8" of piston travel, as the new one was SUPPOSED to.

Some questions:

1. What might I have done wrong with the bench bleed procedure for the new master cylinder?

2. Should I yank it out and continue fiddling with it on the bench until I have it right, or can I do just as well with it installed if I am willing to push enough fluid through the system? It was something of a pain to get it installed, as the threaded fittings didn't quite want to align to get the threads started, btu I'll do whatever is most likely to be successful.

Thanks in advance!

-Roy

 

What? Nobody is going to bite on this one? I'm shocked!

 

I looked but was a little confused about getting stiff resistance while bench bleeding.You should only have the resistance of the inner spring.

 

Well, with the new unit that was all I was able to achieve, but the instructions implied that stiffer resistance would result once bleeding was complete, and that was definitely the case with the old one I removed and tested.

 

Hmm its hard to imagine exactly what resistance your feeling.The bench bleeding lines shouldnt provide much resitance.OK now that the new cylinder is on the car is not bleeding out correctly?

 

The procedure described in the document accompanying the new part differs from any I've seen online. Two plastic threaded plugs came with the master cylinder. The instructions were to thread these plugs into the front and rear brake line ports of the master cylinder prior to starting the bleed. There was therefore no place for fluid (or air) to go but out the 'intake' openings on the floor of each fluid reservoir and up through the fluid in the reservoirs. I'm beginning to think I should have ignored the instructions which came with this part and followed conventional wisdom...

I have not yet attempted to bleed the unit since installing it on the car, because I wanted to ask here on the forum if I will be able to achieve satisfactory results without removing it and properly bench bleeding it first.

 

OK thats makes more sense.If their plugs -no fluid made it down inside to the piston area.That wont work.You have to have the tubes that allow the fluid to flow from those ports and return to the reservoir.Sounds like your master is still full of air.

 

I wondered about that...why the #@%$*&!! would they give such strange instructions? I guess I'll pull it out again and do it properly. Thanks!

 

You can leave it in the car and bench bleed it but be careful you dont spray brake fluid on your paint.I always bench bleed and I'm not 110% sure that you get a 100% full stroke mounted but it should work OK.Maybe someone that has tried to bench bleed while in the car will chime in.

 

I thought the problem with "bench" bleeding in the car is that the master cylinder is sloped upward when installed in the car. Being sloped, air is entrapped air inside the pistons, so bleeding requires the MC being on the bench where it can be bled in a level horizontal position.

My 68 manual brake MC has bleeder screws. This allows the MC to be bled in the car, since the bleeder screws allow air in the piston chamber to bleed out.

I've thought about replacing the MC in my 70 (with no MC Bleeder screws) with a MC with bleeders just for the convenience of bleeding the MC in the car.

Anyone ever thought of replacing their MC with a bleeder screw configured MC?

 

You got a bad rebuilt one, take it back and get another. It's happened before.

 

I have some recent experience here. In the past, when I have had new master cylinders they came with cheap little bench bleeder kits consisting of plastic things that you insert into the outlet holes and connect short pieces of plastic tubing to them, the other ends of which you place in the reservoir. As you push in on the piston you can see the fluid and bubbles coming out the ports and dumped back into the reservoir. Always worked great.

I just bought a new GM reproduction M/C http://www.cssbinc.com/index.asp?Pag...OD&ProdID=1163
It has the bleed screws on the side like 68/70vette described, but it didn't come with a bleeder kit like above. It did come with little plastic caps to seal the outlet ports for shipping. I called the tech line at CSSB, Inc and was told by Luke how to bleed the M/C - I took notes:

With the M/C in the vise and fluid in the reservoirs take the caps out and place your fingers over the outlet ports - this keeps air from entering the ports. While pushing in with a Phillips screwdriver no more than 1", let the air escape by releasing pressure on your fingers. It's kind of messy, but it did the trick. When the air is expelled, he said to hold the caps in the ports and do a series of short, rapid 1/8" strokes
until bubbles no longer appear at the tiny holes in the bottom of the reservoirs.

Once that is done, he said to install the M/C and bleed the brakes, starting at the FRONT! He said it's easier to get the fronts done and get some resistance at the brake pedal before doing the rears. I know this is against conventional wisdom, but remember, the front and rear systems are completely separate. I did it that way with no problems.

Finally, he said when the brakes were completely bled, then bleed the two bleed screws at the M/C.

At the same time I replaced the M/C I replaced the rubber hoses with braided s/s hoses, and added Speed Bleeders to the calipers. I followed Luke's instructions on the M/C bleed, and bled the brakes with just two passes around the car. Easiest bleed job on a C3 that I have ever done.

If I ever do it again, I think I'll buy some brake line that will fit the front and rear M/C ports - they are different sizes - and make my own bench bleed lines, that is, just cut off short pieces that can be bent around into the reservoir. It would be a lot less messy!

Cheers,
Pete

 

Well, I bit the bullet, yanked it out, re-bled on the bench by the conventional technique, Re-installed it, bled all the brake lines (REAR first, 'cause that is what I was always taught), and took her out for a rip. I can lock the wheels when I brake hard, so I guess I was successful!

Thanks again for all your input.

-Roy