bj1k

I converted my 68 over to power brakes with the master/booster combination and pressure bled all of the lines , there is absolutely no air coming out of any of the bleeder screws just pure fluid but I have no brake pedal. It goes all of the way to the floor with no resistance. I bench bled the master cylinder with the bleeding tubes fully submerged with full slow strokes but after about five minutes of doing this the bubbles never fully stopped, so I'm thinking bad master cylinder. Also when you take it over to the car to install it you have to disconnect the bleeding tubes momentarily to screw on the brake lines, so some of the fluid drains before you get it connected, especially since these lines are so short and stiff that it is sometimes difficult to get the threads started before losing the fluid. "Finally to my question" Has anyone ever done the same thing as bench bleeding only doing it on the car by raising the rear of the car slightly so the master is sitting level, taking the two plugs out on the opposite side of the master and inserting the bleed tubes in those ports and the other ends into the fluid in the reservoirs, then slowly pressing the brake pedal until all of the bubbles are gone ? This way you could take the hoses out quickly and replace the plugs without losing much fluid. I'm still thinking this master cylinder is defective since the bubbles never completely stopped when bench bled. What are some more opinions ?

dannyman

You need to jack the car up too high to accomplish this easily.

After you bench bleed the master cylinder you can re-install it on the car and ensure that you replace the reservoir cap before disconnecting the bench bleed lines to re-connect to the brake lines. You should not lose enough fluid to re-introduce air to the master cylinder.

When bench bleeding tilt the master up and down and continue to bench bleed until no evidence of air. This is critical.

dburgjohn

a pedal will go to the floor when there is air in the system, a leak in the system or worn master cyclinder. C3 are known to to be tough to bleed and many use pressure products like a Motive power bleeder.

If you are sure there is no air or leaks, the most likely is a bad MC. The seals would allow the fluid to bypass and not flow out. This can be checked by picking up plugs at your local store. Put them in the two brake line ports and the pedal should be solid if the MC was correctly bench bled.

wcsinx

A bench bleed doesn't have to be perfect. There will always be a little air introduced when you reattach the lines. Yes sometimes the bubbles never stop if you don't get a good seal on those chintzy bleed fittings because the ports actually suck inward when the piston relaxes. And also yes, you can use those ports for bleeding. That's what they're for.

The fact that it's going to the floor with no resistance at all is a bit odd though. Even an MC with bad piston seals will give some resistance. The usual symptom of blown seals is the pedal slowly sinks to the floor with continual pressure. I wonder if the MC wasn't assembled correctly?

72LS1Vette

If you can park the car on a slight incline and jack up the rear so that the master cylinder is level you can bench-bleed the MC on the car. My driveway slopes a little and I didn't have to jack up the rear all that much to get the top of the MC level. Don't guess, put a level on the MC when jacking up the rear of the car.



Rick B.

bj1k

I've been bleeding brakes for over forty years and have to tell you there is nothing more aggravating than working on corvette brakes. I never have any problems at all except with these systems. I have never been able to get all of the air out of the rear calipers without using a pressure bleeder but that is not the problem since I used a pressure bleeder there is absolutely no air in the system from the master cylinder back but still no brake pedal and no resistance. I'm going to try again today to bench bleed the master while on the car using the method that I talked about. ( Using ports on the opposite side of the master. ) Where were their heads when they designed these cars. A high performance car is supposed to have a superior braking system and half of these vettes are running around with the brakes only working half as good as they should since most people just give up when they haven't got all of the air out of the system, works ok until you have to make a panic stop.

...Roger...

Try putting the plugs that the master came with back in and stepping on the brakes,if you have solid pedal its most likely not a bad master.
( The directions with the newer masters say you can actually bench bleed the master using the "plug" and you can (ive done it) but it is not nearly as effective as the tubes. )

wcsinx

You know I've heard this many times on this forum, and I'll never understand it. I'm not trying to come across as condescending, but I've never had a problem with my brakes. And the C3's setup really is not that much different than a modern cars when you get right down to it.

I personally use a combination of the tube-in-jar and gravity bleed method. I crack the bleeder, stick a tight fitting length of tubing on it, put the end of the tube in a jar of brake fluid, give the pedal one full stroke to start the flow, go back to the wheel, and the bubbles are usually done after ~30 seconds, repeat. Poo-poo my method all you want, but I can lock up all 4 wheels with my big toe.

BUT I still am skeptical that air is your problem if you did indeed properly bench bleed. Some air in the system will not cause what's happening to you. Does the pedal feel springy?

...Roger...

I use this method most of time also and if you seal the threads around the bleeders you can also short jab the brake pedal to dislodge stubborn air.

RonR80

I agree with the gravity bleed way I do this all the time , and I gently tap the rear calipers just to get the bubbles out , it's slow but it works. Ron

Rockn-Roll

If you have the m/c connected to a test system i.e. the correct fittings screwed into the output ports which seal against air with long enough clear tubes so you aren't sucking air back in on the backstoke and you are pumping the m/c normally with full reservoirs and still getting air out of the m/c then yes the m/c is bad. But, if your port connections are not sealing then it's OK...all I ever do for bench bleeding is make sure the m/c pumps fluid...I've never had a new or rebuilt m/c which pumps air into the system along with fluid.

As far as bench bleeding...it's not a procedure for getting air out of the system because...well...it's not a system yet. You only need to get all the air out of the system after all the components have been installed.

Grumpy 427

I just did the complete system this past weekend.

Make sure the plastic fittings are tight when bench bleeding, my front one was a bit loose and i had air. I fixed it and kept going, after 3 more pumps on the bench it was solid.

After you install it on the car. Open up the drivers side rear inner bleeder screw, and let gravity take over for an hour or so. I did the same with the front. I have 2 bleeding cups to catch any fluid. Then you need a buddy to sit in the car and get going. It took close to an hour of going corner to corner of the car till we started to get solid fluid to all the bleeder screws.

bj1k

I have solid fluid to all of the bleeder screws from pressure bleeding but the master just refuses to pump on it's own. Sam at Full Throttle Corvettes is calling the manufacturer tomorrow to see why the master keeps making bubbles even after five minutes of bench bleeding. I've bled the system on this corvette a couple other times during replacement of brake parts and have never had this kind of a problem.

wcsinx

It is certainly starting to sound like your piston seals are toast. It doesn't take much pressure to force fluid out of an open bleeder. But if the pedal goes right to the floor with the bleeders closed and no apparent air in the system then the only way that can happen is if you're bypassing fluid.

Oh ... wait...

You said you converted this car to p-brakes, right? Are you sure you got the right booster? There's a long pedal rod and short pedal rod booster under the same part #. The C3's need the long rod version. If you install the short rod version, your pedal will damn near get to the floor before you have adequate braking pressure. Is your brake pedal at the correct height, or do you have to yank it up to make it hit the upper stop?

bj1k

I ordered the master and booster combo for a 68 through Full Throttle Corvette and my booster came with the short rod installed and a long rod included so I called Sam at Full throttle to make sure which one to use . He called the manufacturer to clarify and they told him to use the short rod, but i'm skeptical too since my pedal is only about three inches from the floor when it is fully retracted. It does make sense that the rod is too short but when I bled the master on the car with the clear hoses, I could see the fluid along with air bubbles running through the hoses. It just don't seem to have enough force to push the fluid to the wheels.

bj1k

If someone has power brakes, what is the measurement from your pedal to the floor with the pedal fully retracted ? ( foot off of the pedal )

wcsinx

Well Sam told you wrong. C3's use the long rod. I know this because I went through the exact same problem because the P.O. of my car installed one with the short rod!

I have ZERO DOUBT this is the issue with your car. You can build enough pressure to bleed the system even with that short throw on the piston, but you'll be cleaning out your pants if you ever try to come to panic stop.

Pull the booster, switch to the long rod, and I guaranfreekingtee your problems disappear.

wcsinx

The brake and clutch pedal stops are at nearly the same height. I bet you can yank the pedal up to the stop manually right? And it gives a little *pop* when you pull it up? That pop is the sound of the pedal rod becoming unseated from the booster.

Hook your battery up. What do you bet your brake lights aren't turning off either?

Trust me, it's the booster rod.

...Roger...

Short rod , Long rod !!! Most early C3s use the Long rod , later C3s use the short rod.
The rod length is determined by the MASTER CYLINDER that is used.
The aftermarket suppliers are MIXMATCHING master cylinders and boosters , they are not staying with the correct year application. They are putting for instance using 78 masters on 74 cars. ( This makes buying the correct part by year a nightmare.)
Rod length is something you will have to determine on your own. To start look at the piston in your master, if it has a shallow divot it uses the short rod , if it has a deep hole it uses the long rod.
The last conversion I did I ended up making a rod to maximize pedal height , rod ended up about 1/8 longer than stock long rod length.

bj1k

I did check that before installing the master and the rod did appear to meet where it was supposed to . The rod protruded from the booster about a half inch and the hole in the master was the same depth. ( 1/2 inch ) and as I said when it was installed on the car and bled with the clear hoses, I could see it pushing fluid and bubbles the same as if you were bench bleeding it but the bubbles never stop.