Hello everyone, About four months ago I purchased a 1969 Corvette convertible and have been going through it as the months have passed. My car came to me with braking issues among other things but I've got most of the other issues resolved.
​​​I have replaced the master cylinder, the lines, and the calipers so everything is new. I have bench bled the master cylinder, and tested it with flared fittings to confirm the cylinder works. But the minute I install it and go to bleed the lines everything goes back to an issue. I cannot get a firm pedal feel and I cannot get any decent braking. I have bled all four wheels numerous times. The car is also a manual brake system.

I have tried gravity bleeding and I also own a motive pressure bleeder. The pressure bleeder makes things easy but still not fixing my issue. I do not have any leaks at any of the connections and I am able to bleed at all four wheels which is good news at least.

I have even taken the rear calipers and mounted them vertically to make bleeding them easier and I'm not getting any air from any of the wheels at this point. But I must be missing something. Does anybody have any suggestions and I appreciate your help tremendously.

Thanks
Steve

 

Are you including the hoses when you mention "lines"?

 

Thanks for asking and helping. Yes, I replaced the rubber hoses as well with braided lines. Every connection is super dry, so I do not have any brake fluid seepage, but I don't have any other ideas?

 

The first time I used my Motive bleeder, I had to do the process twice. On the second attempt, I tapped the calipers and junctions with a wrench to dislodge air bubbles. I also used the Motive bleeder bottles, which enabled me to do both sides of a rear caliper, or both front calipers, at the same time. I made sure there was some brake fluid in the bottles at the start, to cover the tube, and I added a tiny amount of blue loctite to the bleed screw threads (NOT to the tapered end that actually makes the seal) to keep air out. Perhaps teflon tape would have been a better choice.

At any rate, the second attempt worked, and subsequent jobs have worked the first time, using these tricks.

I haven't encountered the issue myself, but there is a specific measurement and alignment that must be just right for the master cylinder and plunger. And during any bench bleeding, it may be possible to depress the MC piston too far and damage the seals. Or so I've read on this Forum...

 

When you "bench bled", how far did you shove the MC piston in? If you go beyond a certain point, the MC piston seals (rings) will roll-over. Then on the return stroke the seal is damaged on the casting itself. The MC will never hold pressure. There is over 400 lbs of pressure in the brake system. Your MC may pump up normally, but fails when subject to these kinds of pressures.
The maximum distance for the stroke is 1 3/8". More than that also voids the MC warranty. This type of damage is easy to do on the bench. There is nothing stopping the bench stroke, short of harm, unlike when the MC is mounted on the car.

I have damaged the MC seals myself on a brand new unit. Finally, found out about the maximum stroke which was clearly marked on box of the second MC purchased.
Check out my profile, then photo albums, then bench bleeding for some tips.

 

I'll catch flak for this, but I see no reason to bench bleed your master . Simply fill the reservoir and gravity feed all lines. It may help to slowly work the pedal to assist gravity.

 

What was said above on M/C bleeding: you cannot over-stroke the M/C piston when bleeding it. You should pour out fluid and disassemble the M/C piston to check the condition of the seals. They could be damaged and that would give you a very loose pedal feel. When you have it apart, you can eyeball how much movement of that piston you can tolerate. You could also check how far the piston pin (in the booster) moves when you give it proper amount of pedal movement. Then, use that distance when bench bleeding the M/C.

The easier way to do the bench bleeding is to just assemble the M/C to the booster...with the lines disconnected. Install the M/C bleeder hoses, fill the reservoirs, and use the brake pedal to bleed it. When you see ZERO air bubbles (even little bubbles are bubbles), the M/C is ready to attach the lines.

You should remember that the brake distribution valve (mistaken as a proportional valve) COULD have the shuttle valve inside moved off-center. That could also provide problems, as that valve, when shifted off-center, will shut off one half of the braking system (front or back). So, it might be difficult to get that half properly bled, until the shuttle valve is re-centered.

Once you have done a complete round of brake bleeding, sit in the driver's seat and stomp the hell out of the brake pedal...like you were in a panic stop ON STEROIDS! Do that a couple of times to re-center the valve (if shifted?), then re-bleed all the wheels again. Make sure that you do BOTH BLEEDERS on each of the rear calipers.

If that does not solve your 'soft pedal' problem, there are other things going on and you might need some 'on site' assistance to find the problem. But I think your odds are over 90% that your issue will be solved.

P.S. If you changed your M/C or brake booster, the pin length (between them) may need to be altered, as well. That pin needs to be the SAME LENGTH as the cavity space available for it. It can be up to .010" shorter, but it can not be ANY longer or it will absorb some amount of available pedal travel. If that pin is too short, you may never get to the point of good solid braking with your system...which is also very similar to your symptoms.

 

I appreciate all the feedback so far everyone. you all will probably think I'm crazy but I had read about the seals so I marked my screwdriver at 3/4, one inch and 1.25. I bled first at the 3/4 mark just to get the seals working then stepped up to the one inch mark and then finished with the 1.25 mark. I did use clear tubes as well to make sure no air bubbles were coming through anymore. I might have gone over kill?

I can pull the master cylinder again. But when I did plug it it was rock solid. I'll try to bleed the lines again and see if I can find more air. With all the information everyone has shared with me I have to assume it's air hiding somewhere??

please keep it coming and I can't thank everybody enough

 

What do you know about the distribution block? If the valve is moved from center....and your BRAKE lamp (in the speedometer) is operational, it should be illuminated. You might turn the ignition key to ON (not start) to see if it is lit.

 

The distribution block is also brand new. When I press on the pedal all four calipers squeeze, just not enough and the pedal is not firm.

 

Where did you get the MC? Is it possible they sent you the wrong one for your application?

 

remove the cap and put a level on the master cylinder front to rear. Jack up the rear of the car until the bubble moves towrds the firewall and then bleed the two bleeders on the master.

 

I'm not sure forum rules on vendors but it did come from a large respectable Corvette account.. I got the manual version with deep bore. I do believe it's the correct MC.

I'll try another round and the leveling of the cylinder. It seems it's air or a bad MC?

Thanks everyone!

 

ok Ill get flamed for this one for sure but it fixed the car. I just bought a 71 LT-1 with manual brakes. The brake pedal at purchase was respectable but certainly not great , I would have lived with it but after getting the car home and just checking it over I noticed the front calipers were seeping some fluid. I noticed the car had a new master cylinder on it already when I bought it. So I went and bought front pads and new front calipers. Easy peasy job right , done it a thousand times. Well after replacing the pads and calipers , bleeding, no pedal. bled and bled till the cows came home definitely felt confident no air was in the system , still no pedal. Spent all day doing a one hour job and still no pedal. Next day , went and sat in the car and made an observation. I noticed that if I didn't let the brake pedal come all the way back I would get a pedal , a good pedal. If I held if from traveling it's last 1/8 inch from the top hold it with my foot then hit the pedal again , what do ya know good pedal. So now i'm assuming that whoever replaced the master cylinder probably doesn't have the rod adjusted quite correctly. Being an old man with a bad back the last thing I wanted to do was mess with getting to that rod on the pedal. so what did I do ? I used the adjustment on the brake light switch to stop the pedal from coming back that 1/8 inch I needed. Brake lights work fine and I have no drag on my brakes and have a perfect pedal. So maybe not a proper fix but it fixed it. You may want to see if not letting the brake pedal come back all the way just an 1/8 inch or so now gives you a pedal. If so adjust your brake light switch or do it properly and adjust the rod on the master cylinder going to the pedal. Good luck , stay at it you'll get it.