My new master cylinder didn't have a warning tag in the box to tell me to use short strikes when bench bleeding.
I didn't find out about this until later, and used full-length strokes a number of times, especially when finishing up. I mean, I bottomed the pistons.
I'm having a real problem getting a consistent pedal, and I know the problem is in the rear half of the system.
Is it likely I've damaged a seal?
If so, I'll get a rebuild kit on order.

 

Disconnect the hydraulic lines at the master cylinder and plug the master cylinder with pipe plugs. With the MC full and the lid on, check your pedal. If it's firm your issue is downstream, likely air in the lines. If not, you've isolated your problem to the master cylinder.

 

Never damaged one. Didn't even ponder this until lately and been doing full-length stroke for ages....maybe I'm just lucky but no matter. Do you have a pressure bleeder? Get or make one if you don't. They're really cheap to make with a garden sprayer, some tygon tubing, and an aluminum plate.

 

Been there, done that. Costly mistake. My MC white shipping box was labeled with pale yellow warning. Overlooked it.

Never insert screwdriver more than 1 3/8".

You rolled-over the MC piston seals. Then on the return stroke they hung-up on the casting somewhere.
Start anew. It will never hold pressure.

 

Gerhead74, I'm going to do that next. I have some bolts with the right thread and I'll make little rubber pads to block off the outlets.

carriljc, I've got a Motive Power bleerder, and I'm sure the whole system has no more air in it.

HeadsUP, I'm afraid you're right. There is simply nothing else wrong with the system, and I definitely bottomed-out the piston a number of times, something that the manufacturer says not to do. At least, they tell you that when they remember to put a warning note in the box. Mine didn't have one, and now it's costing me frustration and $$$. I guess Gearhead74's test will tell me one way or another.

 

I over-stroked the MC too. When I learned you were not supposed to, I took it apart and verified the condition of the MC. The over stoke did not damage the MC in my case. You obviously have a problem somewhere and disassembling the MC may be good place to start. I am following you posts closely. Since your original post, there has to be at least three other posts on bleeding brakes. We are missing something.

 

Well, I just finished the test that Gearhead74 suggested, removing the lines from the master cylinder, blocking the ports and then seeing if I had a good pedal. And, I did. A nice high pedal, until one of my cobbled up fittings sprang a leak, but it was the best pedal feel I've had since I started this whole brake project about 3 months ago.
So, the problem is somewhere else, and I'm 90% sure it's in the rear half of the system, because the front brakes lock up, (on gravel), easily, while the rears seem to do almost nothing.
I'm pretty sure I've let some air into the master cylinder while fooling around with this, so I'm going to start from scratch. Remove the master cyl, carefully bench bleed it, and then gravity bleed the whole system for a good long while. I've just installed Wilwood callipers on the rear, and I'm going to unbolt them and tilt them up each way so the bleeders are facing straight up.
In the next couple of days, I'll post what happens, and I hope it's something useful, because after this, I am completely out of ideas.

 

You must only block one side at a time.
Leave one brake line connected and plug the other.
Then reverse and check the pedal after each test.

 

"only block one side at a time"
Unless there is a leak downstream from the MC. Then you may blame a perfectly good MC.

I think its better to block both MC ports when testing. This eliminates any other variables.

Also , use great caution when blocking off those ports. Deep inside is a cast flare. You do not want to damage that using a hardware bolt..

It is impossible to keep air bubbles out of the system when removing one or both lines from the MC. So, a full bleed is in order.

 

Hi HeadsU.P.
Yes, you're correct to first check the lines and calipers for leaks.

The master cylinder has a front and rear reservoir
if the front seal in the M/C is bad and plugged and the rear line is connected you will get a rock-solid pedal.
If the rear seal in the M/C is bad and plugged and the front connected the pedal will go down.

Yes, you have to bleed the whole system the same if you rebuild or replace the M/C.
After the final assembly hold a rag under the M/C to the brake line and open a half-turn while a helper slowly presses the pedal down. This will purge any are up there.

 

With all due respect:
I never said: check the lines and calipers for leaks. Anything past that MC port fitting, is downstream to me
Must have been the fella four or five barstools down that said that.

What I am getting at is, when testing the piston seals for failure in a MC and to be able to hold pressure, its best to block both front & rear ports on the MC. That eliminates testing anything else in the brake system beyond those MC ports. Its a solo test for the MC only.

Leave one brake line attached you say? What if there is a leak down by the firewall in the brake line itself or at a fitting at the Proportioning valve? This will allow the pedal to go to the floor and falsely indicate maybe the MC is bad. When in fact, the MC could be ok.

I realize the concept for dual masters was to always have some brakes in the event of a failure, front or back. But the Proportioning Valve does that.

 

Ok, as I posted earlier, I blocked off both ports and had a high and firm pedal.
Off came the master cylinder this morning to be carefully bench-bled, which is finished. While doing it, I tilted it side-to-side, up and down lengthwise, didn't exceed a 1-1/4" stroke, and didn't quit until the last tiny bubble was out.
I'm in the process of making a slightly longer push rod between the booster and the master cylinder, because the original seemed too short. When it was adjusted correctly for clearance, there were only 4 or 5 threads in the rod coming out of the booster. I should have the rod made and installed today. Then, re-install the master cyl, and crack each line for a second with somebody, (wife), slowly pushing on the pedal, and tightening it up before it hits bottom, to eliminate any last bit of air there.
Next steps are to start a long, slow, low pressure bleed, 2-3 PSI, dismounting the new Wilwood calipers to have them absolutely vertical, and letting each bleeder flow for about 15 minutes, in the correct order, (according to Wilwood), RR outer, (yes, outer), RR inner, LR outer, LR inner, RF, and finally LF, all the while tapping away with a rubber mallet on the calipers, lines, the proportioning valve, and jiggling the flex hoses.
I'll keep you posted.

 

I agree that you need to test by blocking each outlet individually. The master is basically 2 separate hydraulic pumps in series mechanically so only one pump needs to work to stop the piston.

 

Sorry, but I disagree.
No matter what, if the master cylinder holds a high firm pedal with both ports blocked, the problem is not in the master cylinder, and you can at least eliminate that. If, for example, you block off the front port, and the pedal goes to the floor, is the fault within the master cylinder, or a problem with the rear part of the system? There's no way to know which.
I now have a high and firm pedal with both ports blocked. With the lines hooked up, I have a low and soft pedal, with the front brakes locking up quickly, and the rears doing almost nothing. To me, this means the problem is definitely in the rear part of the system, and I can move on to eliminate one thing after another until I get there.

 

I'm not convinced that would be true. I'd have to see the insides of a C3 master to see how the two pistons work together. In the past, On other cars, I have had rear brake issue where the front brakes are well bled and couldn't pump bleed the rears because the pedal would hardly move.

 

I'd get one of these to verify the pushrod length is correct. https://www.ecklerscorvette.com/1968...All%20Products

 

hdeyong,

Use caution when fiddling with those booster rods. You want a 0.060 gap there I believe. Any tighter and the brakes will drag all the time. There will be just enough pedal weight to apply pressure to the MC. The caliper pistons / pads must have enough room to retract. Which is not do-able without a booster rod gap.

And some year models have two optional holes for the booster rod & pedal assembly. Some have adjustable rods. Some had no adjustments.

 

3X2 and HeadsUp, what I've done is to adjust the new push rod I made until I had a little more than 1mm, (about 0.050), of clearance. I did this by adjusting the rod, and sliding the master cylinder into position until it hit the rod and the booster mounting surface at the same time, and then backing the rod off 1mm.
Tomorrow, I'm going to have my very patient and understanding wife, (of 44 years), to slowly push the pedal while I crack open a line fitting, closing it long before the pedal bottoms out. I'll do this a couple of times for each line. Apparently, this can help get any last bits of air out of the junction where the lines come out of the master cylinder.
Then starts the long, slow bleed.