Hi all,

I just installed new lone star caliper o-ring calipers, new master cylinder, proportioning valve, new rubber hoses, and front hard lines. Bench bled the MC, raised the rear up high enough to have the rear chamber higher than the front and have verified no bubbles left coming into the MC. I have pedal bled, pressure bled, and gravity bled. I see no leaks. Pedal gets nice and firm while bleeding and with the engine off and all seems good to go. But when I start her up and press on the pedal, it goes almost to the floor again. I have some brake just way to much pedal travel but only when the car's running. Not sure what to try next. Looking for ideas at this point as I don't have much hair left to pull out...

 

You have done just about everything I did with a problem Bronco that I had to deal with. I even had a pressure bleeder. If you've done all 4 rubber hoses and pressure bled, then the only thing I can think of (assuming all your equipment is "good") is to check the tightness of your fittings. On my Bronco which I could not get to have a good pedal for months---the leak/weep was so slight that I could barely see, or feel dampness, even after seveal days..... It was hard for me to believe that would make my pedal so crappy. I finally ended up TORQUING THE **** out of the suspected fittings (mine were on Master Cylinder) and I immediately was able to get a good pedal.
If you have changed ALL of your rubber hoses and all that other equipment, then go back under there and really check the tightness of all the fittings you disturbed, and any other that may have been disturbed by tightening it's neighbors. SO,if there are any rubber hoses you did not change, then change those, and RETIGHTEN all the fittings and try again.
On another car the rubber hose just decided to not allow proper bleeding after just chaning a pad ..... I didn't even do anything to the hose other than move it around a bit.... it was hard for me to believe that movement no longer allowed that hose to allow proper bleeding......but I finally changed it and immediately (again) had a hard brake pedal.
Just my 2 cents.

 

Yeah I replaced all the hoses and there is no weeping from them. Pounded on the calipers and MC with a rubber mallet while bleeding. I’ve tightened every connection to the point of anymore and they’ll start to strip. I’m going to remove the MC lines and try bleeding it again. Maybe I missed something but I don’t understand how I can have firm pedal without boost but once boosted it’s crap again. Ugh….

 

Corvettes have an enormously long amount of brake pedal travel.
3.8" is almost 4 inches and very close to the floor but if the wheels lock up on a panic stop you just have to get used to it.
You did it all, maybe it's as good as they get.

 

I use a power bleeder and you already said you used the hammer ,, have you adjusted the rod that goes into the master did you look at the old one before you installed the new one and one more question did the brakes work at any capacity before you replaced the parts
dont pull your hair out its not worth stressing out its usually something stupid

 

Yep I have the motive pressure bleeder and have run plenty of fluids thru it. I did look at the rod but it's not adjustable. This is on an '81 if that matters.

The brakes work but not to the point of lock up. And yeah they worked ok before I started. They were never rock hard but worked decently enough but one of my rears started weeping a bit and I’m pretty sure the master was original. I pulled it apart after getting the new one and it was gunky in the bore so it was time. I know it’s not rocket science and have bled many systems before but I swear corvette brake systems are cursed. Why would it feel firm without boost but once running it feels spongy with way more travel? Seems odd to me. How would you ever know if they are bled without having to start the engine?

I’m going to isolate the two systems and see if it might be the front or rear causing the issue. My plan is to put a short crimped line on the rear MC port and leave just the front connected and see what I get. I’ll swap the front and rear and test. Maybe that will give me some indication. If that fails, I’ll put crimped lines on both ports and test just the MC with the boost on. That should give a very firm pedal if working correctly I would assume. Man this is a pia…

 

Interesting. Thanks for posting. As I read this, since I have power brakes, the check is done without boost and from what I can tell my behavior is pretty much spot on. Maybe 1.5-2" of travel and the pedal is very firm so from what I gather, it should be working properly. However, one the engine's running, it loses the firm pedal and the travel is much further. I don't get it...

 

A properly working booster when purged of vacuum will give a rock hard pedal'
once the engine is started the booster's vacuum starts to increase and the pedal will drop a little.
That's how it works, You can always use your Vette and re-bleed down the road and see if and air comes out.

 

sci85,
Your post # 6 is spot on.
You need to "isolate" the new Master to see if that is an issue or its fine. However, I wouldn't bother testing front / rear separate. Just install both your test lines on both ports. You should be able to stand on the brake pedal once the fluid enters the test lines and is blocked. If not, look no further. If so, move on.
You will need two test lines. The front & rear MC ports are not only different diameter, but different thread pitch.

Guess what? You have now introduced air into the system. You know what that means.

 

yep I already have the proper lines. I figured it might be helpful to know if one side or the other is a problem first but yeah, I’m not looking forward to introducing air into the lines again but at this point I need to start eliminating something or I’ll go mad!

 

You are on the right track, if you have the hoses and correct fittings a crimped line works.

In the HELP section, they have bleeder kits and some have a 1/4 X 20 and a 9/16 X 18 plugs included.

Leave 1 line connected and 1 plugged.
Then switch the plug and the line and the pedal should be rock-hard in both cases and hold the pressure for 30 seconds or more.

 

To prevent rebleeding have a helper very gently press the pedal down while you with a rag catch the fluid and quickly attach the line.
You might get lucky!

 

I’m going to shed a tear when I have to open those lines again…of all the work I’ve done on her, this is been the biggest pai to date…

 

Wait until the state tells you your parking brake has to work to pass inspection.

If there is air in the MC, that's a problem, good luck. If there is air elsewhere, perhaps try tapping around with a wrench as the Motive does its thing. Thats all it took with my 79. After the second bleed, with tapping, I can lock up all 4 calipers.

 

Parking brake: That's a separate can of worms.

 

Yup! But a much bigger PIA, especially with today's ill-fitting parts.

 

There is nothing that really goes bad from a hardware (behind the rotor) standpoint, most always a good cleaning and sand blasting will bring the OEM parts back to a useable condition. I try to stay away from repo junk and dumpster dive the internet for good used OEM when I can, the repo stuff is such a crap shoot for Fit, Form and Function.

 

Well after messing with bleeding for days, I decided to test bleeding the MC again but using steel lines this time. I grabbed the old crusty master and put it in the vice, hooked up the bleeder lines and pumped and pumped. It literally took me 20min of pumping before finally no air bubbles. Then I removed the bleeder lines and hooked up my crimped lines. Plunger moved maybe a 1/4” and no more. Rock hard. So, at this point I’m figuring I didn’t bleed the new MC near enough. Jeez it takes a long time to get that air out. Will post results of doing the above on the new MC in the next few days. Thanks all.

 

About 7-8 strokes on the level, in the bench vise. Another 7-8 strokes with the MC tipped downward, then 7-8 tipped upwards.

If air bubbles continue to surface, I would suspect the piston seal at the rear is letting air in.

Keep the test lines installed as you bolt the MC to the car. Then hook up your brake lines, re-re-re-re-re-bleed.
You should be good-to-go.

Just a quick note here: The pressure you exert on the MC with your screwdriver is a fraction of what the brake pedal will exert. (leverage)
So, it is still possible any MC could be defective without any shown symptoms in the vise. The MC will have to handle as much as 400 lbs of pressure.

 

Agreed. After bleeding the old one in the vise, I don’t think I did as good a job as I could have on the news one. I’ll bleed it again and do a better job this time. Then I’ll hook up the crimped lines and test it with and without boost.