master cylinder problem?

[Zed Head]

Here is one possibility, based on how I think the master cylinder pistons are set up. I believe that holes to the reservoirs are supposed to seal off after a very small amount of piston movement.

Did you pull the rod from the master cylinder back from the firewall to get it to connect to the pedal? Maybe you are pulling the pistons back in to the bore, loading up with fluid that has to come back out. The fluid squirts until the hole gets sealed again. The rod end should be loose enough to wiggle back and forth a little bit. Does the fluid squirt at the beginning of stroke, then stop as pedal pressure builds?

I haven't a master cylinder apart so don't really know if that's possible, but if it is, it might be plausible.

Edit - just realized that if the MC is defective, it should squirt during bench bleeding too.

Edit 2 - One more thing - if you connected the rod to the pedal, then pulled the pedal back to connect the spring, you could get the same fluid loading effect (if it's possible).

Edited November 14, 2010 by Zed Head

[ta240]

I sent them pictures of the MC in action and they sent a return label. I haven't gotten other yet as I just put my old one back on it.

anyone ever rebuild their MC? I've done calipers and wheel cylinders in the past but my comfort level with doing the less expensive repair on brake parts isn't very high anymore

[ta240]

okay, I must be doing something wrong. I ordered a new (non-reman) master cylinder and installed it this weekend. And it is doing the same thing. I have a video clip of it at: http://240z.me/mc.html

I bench bled it and even used a 'tool' they sold that is basically a syringe with a rubber cap on it that pushes and pulls fluid through the master cylinder from the outlet port at the bottom. And did that till no bubbles came through. Here's a picture of the tool: http://kingtuttonline.com/~rjays2/images/mpbP1012869-lg.jpg

Last time I bench bled it the regular way.

looking back at the comments on the prior problem, the rod rests in the booster so it doesn't pull the pistons back. But if I start with the pedal down a bit and press hard it won't shoot fluid out so it looks like it isn't sealing up fast enough once the pedal starts to travel. After trying a few different things I think if I adjusted the push rod out of the booster and rod that connects to the pedal I could move it past that point, however it would have to be a combined adjustment of over a half inch and that seems like a lot of adjustment and would reduce overall pedal travel a noticeable amount.

Edited January 23, 2012 by ta240

[beermanpete]

The brake fluid squirting up out of the reservoir is normal. When the master cylinder is at rest there is a "vent" passage that is open between the reservoir and the bore in the master. This allows brake fluid into the master cylinder bore. When you press the brake pedal and the piston moves it forces fluid out the cylinder, through the vent passage back into the reservoir until the seal goes past the vent and closes the master cylinder. At this point the brake pressure in the master and wheel cylinders starts to rise applying the brakes. The reason you are seeing the squirt is because you are missing the filter screens that are normally installed in the reservoirs to keep debris out of the master cylinder. The screens will diffuse the squirt so you don't see it.

You don't want to adjust the push rods to hold the pistons down to prevent the squirt. If you do this the brakes will not release. You must have some free play in the push rod between the pedal and booster, and between the booster and the master. Proper adjustment is covered on the service manual.

[ta240]

The reason you are seeing the squirt is because you are missing the filter screens that are normally installed in the reservoirs to keep debris out of the master cylinder. The screens will diffuse the squirt so you don't see it.

thank you! That makes perfect sense. And completely explains why some master cylinders had that 'problem' for me and some didn't. My old screen from my old MC fit the new one and no more 'problem'.

Thanks!

[ta240]

and here is why it is often better to buy parts from places that know and care. The online place I ordered it from replied to my question about the brake fluid with a cut and paste answer of "In regards to the question, I would suggest taking your vehicle to a certified mechanic for a proper diagnosis before buying and having parts installed that may not (solve/be) the problem. We want to ensure that you have your concern addressed and resolved the first time!"

hmmm, so the brake fluid leaking out the back of the master cylinder into my booster might not have meant that I needed to replace the master cylinder?

My email to them had nothing to do with the new part not fixing a problem it had to do with it having an entirely new 'problem'. Granted it turned out not to be a problem but still, what a blow off in dealing with the issue. And if they'd include the little strainers in their new part it wouldn't have been an issue at all.

I've had questions about products from Motorsport Auto before and never got an answer like that. I guess you get what you pay for.

Edited January 24, 2012 by ta240