Your thoughts on bottoming master cyl when bleeding?

[BLINGMW]

Just wondering what you all have heard, do you think it's an urban legend that bottoming an older master cyl completly when bleeding brakes could lead to seal failure? It makes sense I guess, that the section of the bore that isn't normally used could build up some surface rust or some such gunk and chew up the seal when you bottom it out.

If there is some truth to it, are we talking about REALLY old neglected systems, or is it something to think about a normally maintained street car? Certianly something I shouldn't worry about on my frequently bled track car with fairly new master cyl right? ....RIGHT?

Discuss

[.RJ]

Yes, it happens and I've ruined 2 master cylinders this way. Put a w00d block under the pedal. On a newer car you are probably fine.

[Andy]

I, too, have screwed up a master cylinder this way.

[Mike]

hrmph... maybe that is why my first master went it was too small anyway... good info!

[Evan]

ive always bottomed the pedal and never had a problem

[Feersty]

Doesn't the idea of having someone constantly pouring fluid into the reservoir prevent this from happening?

I would think you are obligated to have a buddy or two assist while you bleed the brakes.

[Mike]

Bottom out the pedal is not running out of fluid... it is... bottoming out the pedal, which sends the master cylinder through the entirety of its possible motion. This doesn't normally happen since there is usually fluid pressure stopping it. Since it never does that there can be buildup of just... shit... When you run the rubber seals of the master cylinder over such gunk and shit, it can tear or dirty the seal, which would pretty much ruin its effectiveness.

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[Dave]

interesting, never thought about this before. I usually just give it good pumps, never paid enough attention to whether it is hitting the firewall or not though.

[BLINGMW]

Interesting! I didn't know you guys had personal experience with this. Glad to see my superstitions were founded. I've done the wood block on cars with old masters, that works well. I wonder how often you need to bleed in order to not worry about bottoming it? Evan, you've bottomed the cyls on the old miatas and such?

[Mike]

chan, i regularly bottomed the cyl on my 93 hatch... it did give way after a while, but i don't know if that was the cause or if it was the fact it was 12 years old and had like 200k miles on it?

[Jeff]

I have honestly never heard of this problem. I have been involved in bleeding brakes on many models and many ages of cars and I have never been warned of...or seen this. When I'm at the garage in the summer I am brake bleeding bitch. "Just pump er' slow and all the way to the floor!" It might also depend on the style of MC you are dealing with. Some cars (Like that damned Chrysler Maserati thing) have funky masters that incorperate ABS stuff and other things. Might also depend on the size of the vac booster. Just some thoughts.

[.RJ]

All Master cyl's are fundamentaly the same - a pushrod, pushes a piston, which seals inside the bore of the cylinder. Most of the time, the piston has a limited range of motion, and if you dont flush brakes for 5 years you can get some corrosion inside the cylinder. When you now push it all the way to the floor, it can damage the seal.

I'm suprised your dealership doesnt use a pressure/vacuum flushing system.

[Jeff]

All Master cyl's are fundamentaly the same

All m/cs are the same on the basic level true, but have you ever worked on one that has all that ABS junk connected to it? Its like little spring valves connected to the bottom of the main length of the cylinder under the reservoir. When you take it apart it looks like a master cylinder that runs though the center of an automatic transmission valve body. The newer systems are nothing like it. It has a high pressure tank thing attached to the top and all kinds of crazy stuff. Thats why we never took the one on the Maz apartthe service manual scared us away. The point is not lost on me though, junk in the end of the cylinder Fs up the sealsense is made.

I'm surprised your dealership doesnt use a pressure/vacuum flushing system.

I was getting ready to say we donÔÇÖt but then I googled ÔÇ£break bleeder systemÔÇØ and the thing I found looks just like one of the shiny new tools Hal (my uncle) got over the past few months.



He had been upgrading all the equipment over the past year and has spent a ton of money on new goodies to make jobs easier and faster. Just to show how far we were getting behind, we just got our first trans flush system last month and our tire mounting machine doesnÔÇÖt got any larger then 17in. We have random machines...like an on the car brake lathe. I am never down there anymore; never get to see the new stuff.

[Evan]

Evan, you've bottomed the cyls on the old miatas and
such?

yah

[Nick325is]

I tried the pressure bleeder approach on my car using a motive power bleeder. I had no luck using the thing. I followed the directions that came with it and could only get a small stream of brake fluid to come out.
I ended up using the traditional pump the pedel method which I think works the best.


Nick

[.RJ]

I rebuilt my spare calipers and put them on the car on sunday, and did the usual gravity bleed method with no problems. I just found other things to work on while the bleeders were open (did 2 calipers at a time).

[Andy]

My master cylinder is on its way out. Fluid is slipping past the seals. I've had to bleed the brakes the past three weeks in a row for various screw ups fromthe folks who sent me my SS lines. The MC had 134,xxx on it so I guess I can't complain but now I have to find a place that sells ATE gold, or motul or the like. I really do not want to drive all the way out to OGracing and olympus only has ATE blue for 20bucks. Anyonegot any ideas about whereI can find some good brake fluid.

*Edit* Sorry, I should've said that I was looking for some to p/u in NoVa. I just don't want to drive all the way to OGRacing.

[JohnC]

CDOC here in Charlottesville usually has a good price. Call them if you have to, ask for the "racer" price.

[Maengelito]

CDOC here in Charlottesville usually has a good price. Call them if you have to, ask for the "racer" price.

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i believe typ 200 and super blue are 2 for $20

[Jeff]

What is so special about super blue? can it be used in cars that call for DOT3 fluid?