ettev

I installed power brakes on my '70 this winter. Never had a problem with air in the lines prior to that. As part of the PB install naturally I had the master cylinder off as well as the lines down to the proportioning valve. Calipers remained in place.

I installed an original/correct PB master cylinder which was rebuilt. I was told it only was honed and not stainless steel sleeved.

I've bled the rear brakes now 4 times and got a good pedal each time with no signs of air after several 2-person manual bleeding procedures. I drive the car for no more than 50 miles around town and the pedal is to the floor. Each time it's the same thing......air in the rear calipers only. Rotor runout is .004" on the drivers rear and .009" on the passenger rear. Can this cause that much air so quickly in the system? The rotors were in the same position when I had manual brakes and never had an air problem. Can there be that much air in the system from when I had it apart for the PB install? Bad master cylinder?

There are no signs of leaks anywhere along the brake system.

Aaaaaarrrrrrrrrrrgggggggggggghhhhhhhhhhh !

I got o-ring upgrade kits for the rear calipers but I'm wondering if that will cure the problem?

Suggestions anyone?

wombvette

Yes that much run out will cause the air. The O rings should do the trick, although I have never used them. I always get the run out corrected.

ettev

Why wouldn't I have had air before the conversion to PB's? If there's runout now, it was there before.

wacked

I feel you, I'm battling with air in my lines to. I bleed my system bunches of times.

silver77vette

Had the same problem with my 77 when I installed all my SS lines, finally power bled them. No problems now and have great pedal feel. :thumbs:

wombvette

The PB system is more sensitive to air in the system. You may have had some before, but not enough to cause a problem. In any case, it is obvious that you have the problem and need to correct it.

jacobsed

What is power bleeding and why does air get in the system because of run out?

cardo0

It could how you bleed the brakes ettev. I use a jar type one man bleeder and have no problems as long as the end of bleed tube is always covered with fluid in the jar and the reservoirs up in the master cyl never goes low out the bottom. And that master cyl needs to be bleed before install and can cause problems if not bleed properly. I think you can bleed the master cyl installed but I don't recall how and you may find this with a search. Ecklers is now selling a power bleeder pump kit for about $55. This might be a little faster.
In the past I tried a vacuum pump and I found it really pulls too hard . It was pulling air right past the new caliper seals . Tried the bleeder jar and never looked back.
BTW I'm thinking about removing both my pb and ps. I only have a small bolck and don't expect any parallel parking. I used to have a heavy GTO with manual drum breaks and it stopped okay. So I guess I'm asking why the change? :seeya

71coupe

Since this happened after you replaced the master cyl, I would say it's bad. The pressure bleeder kit is the best way to go IMO. Mid America sells one that I'm thinking about getting.

ettev

The problem is in the passenger rear where I have the .009" runout. But how much air could this suck into the system? After a ride I'll let the car cool down and bleed the rears. I can go through quarts of fluid and still see air in the clear bleed hose.

How many places along the entire system are capable of "sucking" air into it? Partially opened bleeders on master cylinder or calipers; loose fitting someplace; hole in the one of the lines; flap seal on the caliper piston; bad o-ring seal between caliper halves. Thats all I can think of. Wouldn't I see evidence of a leak with all of them in the way of dripping fluid?

I've also just installed Russell Speed Bleeders which seem to work as advertised.

Thinking about the master cylinder, when I bleed the system I seem to get a hard pedal, or at least as hard as it gets. Is it possible that the heat from the engine compartment is somehow affecting the internals if the master cylinder in such a manner as to allow expansion thus permitting air to be drawn in? I'm stumped. There is no sign of air in the master cylinder reservoirs when I'm bleeding.


[Modified by ettev, 10:00 AM 5/2/2004]

Today's problem is..

I had a similar problem on an old van I had when I installed a new master cylinder, I would get a hard pedal, then drive it for a bit and it would get spongy. The problem turned out to be the internals in the new master cylinder. I would start there as it may be leaking internally.

ettev

But if the master cylinder is the problem why wouldn't I get air in the drivers side rear caliper as well? Wouldn't both rear calipers be equally affected? That's why I'm kind of leaning toward something wrong in/around the passenger caliper area.


[Modified by ettev, 10:55 AM 5/2/2004]

wombvette

ettev, I dont know how to be more emphatic, without saying something that may make you mad. I have worked on these things for more than 30 years. I have seen many people change MC and all sorts of things chasing this problem. Believe me .009 is enough to pump air. That air is pumped by the seals in the calipers. This is evidenced by your only having problems with that caliper. If you dont want to continue to chase your tail, FIX THE RUNNOUT, and you will have no more trouble.

ettev

Well, I bled the rear again and got as much air as fluid from the right rear. Disconnected the brake line from the caliper and bled it through a clear hose and, viola, no air! That eliminates the master cylinder from the mix.

Verified runout as being .009". Took the caliper off only to find that it was leaking ever so slightly from the inboard forward piston (yes, it is a stainless steel sleeved caliper from Dr. Vette). No visible signs of nicks or tears in the original lip piston seal, but it was obviously sucking air in.

Played with soda can shims and got my runout down to .002". Also split the caliper and used 1200 sand paper to clean up the piston bores. Installed new o-ring pistons, dust seals, and passage o-ring.

Put it back together, bled the brakes. Hard as a rock! Couldn't do too many miles of test driving due to impending thunderstorm. I'm betting I've fixed the problem. Took all afternoon, but I'm happy. When I get time I'll upgrade the drivers rear caliper with the o-ring pistons.

I was a doubter, but rotor runout can in fact cause much :banghead: :banghead: :banghead:.




[Modified by ettev, 8:50 PM 5/2/2004]

Turbo-Jet

FYI- before I replaced my brakes, I had about .012 runout on both rear rotors and never had a problem with air... just a squeaky pad pin.


[Modified by Turbo-Jet, 9:41 PM 5/3/2004]

Turbo-Jet

:confused: huh?

toolking

I had the same problem on my 72. Several bleed jobs and 2 sets of caliper later we finally did the runout and no more problems.

Good luck