Project update

[ChrisMiami]

And, for what it's worth, it hasn't been 2 years since my last update, it's "only" been about 9 months.

Oh man, that math problem of mine shows up every time I measure twice, too! I'll probably end up having to redo a lotta stuff because of it!

Chris

[MaineVette]

The weather for this weekend is looking pretty good - a balmy 40 degrees! I have some chores to take care of today but tomorrow I'm hoping to get out in the garage to finish bleeding my brakes - the last step before taking a test drive. Assuming my efforts are successful, and I can dig my way through the snowbank in front of the garage door (have I mentioned I hate winter), I might even get to take the Vette for a ride down the driveway...

Of course I just completely jinxed it...

[MaineVette]

Well today started out pretty good. It was sunny, it was warm, I had the day off, and there was nothing but working on the Corvette on my to-do list. I was feeling pretty good that I was actually going to drive the vette today (spoiler alert - I didn't). All I had to do was finish bleeding the brakes.

The last time I worked on the car I was bleeding the brakes but didn't have much luck - the brake pedal was soft and spongy. After a couple hours I ended up giving up.

After giving it some thought I decided to buy a vacuum bleeder from harbor freight. This morning I went out in the garage, jacked up the rear end of the car about 15" to help the process along, and started bleeding the brakes with teh vacuum bleeder. It seemed to work OK in that it pulled a decent vacuum. The problem was that air kept getting sucked in from around the threads of the bleed screws. Obviously that's not ideal when you're trying to tell if there's air in the lines or not. Basically they're cheapo bleed valves - they don't have any o-rings or other seal. I was going to apply Teflon tape to the threads but was worried some of the tape would get loose and mess up the system. I even tried applying some silicon around the perimeter of the bleed screw to prevent air infiltration but that didn't help either. I started thinking the vacuum was pulling air past the o-rings of the calipers.

After messing around for most of the day I just gave up. I had pulled about 3 quarts of fluid through the system and, no matter what I did, I just couldn't get a firm pedal. The brake pressure just wasn't there to stop the car.

I know my sequence was correct (right rear, outer then inner; left rear outer then inner; right front; left front) I think the vacuum bleeder just isn't the way to go. The only thing I can figure is that it was pulling air past the o-rings in the calipers.

I even tried gravity bleeding but the fluid just wouldn't move - it would have taken days.

My next attempt will be to pressure bleed the system. I'm debating whether I buy a pressure bleeder or try to jury rig something. Those pressure bleeders are expensive!

To be continued...

[rbryce1]

I have a pressure bleeder that is a small reservoir with a small plunger type air pump that clamps onto the top of the master cylinder. You put extra brake fluid in the reservoir, pressurize it with the small hand pump, and bleed the brakes RR, LR, RF,LR like always. Theory is sound. The air pressure pushes the fluid until you get no air, then move on to the next one. Just have to make sure the reservoir does not run out of fluid.

In reality, it SUX more than it blows. It is near impossible to keep a good seal on the top of the master cylinder, and brake fluid leaks everywhere. You can try it, but I hope you have better luck than I did. I don't recommend it.

When Van Steel replaced my front upper and lower control arms and both rear control arms, they bled my brakes. They have a little vacuum gun they use on each caliper. It only pulls enough vacuum to move the fluid, not enough to have any air leak past threads or piston seals. There was no reservoir on the master, they just kept it full of fluid as they went. They even left the master cylinder lid off while the did it so they could see the fluid level. They did my whole car in about a half hour total.

I don't see why you need so much of a vacuum to pull the fluid. Is it possible you have a bad flow restriction, maybe in your proportioning valve???

[MaineVette]

I've been debating pulling the proportioning valve but haven't yet since I know it'll be a mess. I had noticed the fluid was moving slower than expected through the lines but it was similar front and rear. Therefore I figured the slow fluid movement was more a function of the master cylinder than the proportioning valve.

Are the proportioning valves even servicable?

[vito]

Sorry I am late to the party. Tim, I don't believe you have a proportioning valve, it is a switched distribution block. GM P/N Switch 3980797. Also if you look into the block you will notice that the holes for the front brakes are larger than for the rear. Also the front calipers are larger than the rears. ie pistons are larger. You should have more flow in the front than the rear, ie larger tubing = more volume.

Bleed screws. I had this problem as well and went back to the originals. problem went away. I think that the screws now are more finely cut.

Teflon tape....you crazy....I have some soaking in brake fluid now and will take a picture tomorrow. NO A/C tech has that in their tool bags. A/C oil eats it.

Remember the chit that I had in my calipers...I have new kits for the fronts are well as the master cylinder.

So it all starts with the master cylinder.

PS. I hate working on brakes too

[MaineVette]

Vito - Thanks for the input on the proportioning valve. You're absolutely right It's not truly a proportioning valve that "distributes" fluid. I realized that just after reading your post and did some research trying to figure out exactly what this thing does. Basically I concluded the valve is basically a fail safe that closes off the front or rear brakes in the event of a blowout. The significant pressure difference causes a plunger in the valve to move either up or down and block off flow from the front or rear master cylinder feed. When the plunger moves it also activates a switch in the valve which causes the brake warning light to come on. Pretty neat setup actually.

I would love to install my old bleed screws but they were returned with the old calipers so I could get my core charge back. I may buy some new ones from Summit.

My next step will be to verify the plunger in the valve isn't off center. I can do this pretty easily with my ohmmeter. If that checks out I'm going to try bleeding the brakes one last time.

Bob - I know exactly what you mean about pressure bleeders making a huge mess with these old master cylinders. They're much easier/neater to use with new systems that have screw top reservoirs. I've decided to try something a bit different. I'm going drill a hole in my master cylinder cover and gasket and attach an air fitting to it. I'll seal it up real good and attach my air hose to it (thinking of using my trigger operated tire filler). I'll then fill the master cylinder about 3/4 full with fluid, put the cover on, hold it down with the bails, and put about 10 pounds of pressure in the system. I know that, when the master cylinder reservoir is full, I can fill the bottle on my vacuum bleeder twice. So, I'll simply attach a hose to the bleed screw, crack it open, and let it drain into my bleeder bottle until it's full. I'll then close the bleed screw, depressurize, top of the master cylinder, and repeat. This should minimize excess fluid running all over - at least that's the idea. The new cover and gasket run about $17 so it's cheap enough to replace them. Much cheaper than $100 for a pressure bleeder system.

If that doesn't work I'm going to have to do a system tear down like Vito did to check things out (I need to reread your posts on teh subject Vito, there was some good info on there). To my mind it just shouldn't be this hard to bleed the brakes. It makes me think something isn't right. The system sat for a long time without fluid in it which worries me. I'm really not excited at the thought of pulling the calipers, master cylinder and proportioning valve but it's pretty important the brakes work right - gotta slow down from those burnouts somehow!

I'll keep you guys posted. I have some time today but have a family get together this afternoon. I also have tomorrow off and may get out in the garage then too. Fingers crossed that I don't need to tear the entire system down.

[MaineVette]

Well, here's an update. I checked the proportioning valve and verified it wasn't in fault mode. With that verified I made my makeshift pressure bleeder (I'll post some pictures later) an hooked it up with about 10 pounds of pressure. I went to the rear drivers side caliper, hooked up my clear bleed line and cracked the bleed screw. Fluid started coming out slowly but then it slowed down quite a bit, and then stopped after getting about of ounce of fluid out. I pumped the brake pedal which got the fluid going again but, after a moment, it stopped again. Obviously something isn't right. I messed around with it enough to get about a half cup of fluid out of the system and, during that time, I didn't seen any bubbles come out.

So, I decided to bite the bullet and pulled the proportioning valve. Man what a pain in the arse that is with the engine in the car, the access to the fittings is terrible. Anyway, Once I got the proportioning valve off I checked it out and everything looks ok. I ran some carb cleaner and compressed air through it but didn't see any debris or junk come out and the air and fluid ran through it very easily. I don't think the valve is the problem.

I'm starting to wonder if this is a master cylinder issue - it's the only other component I can think of that would cause this type of problem. I bought a rebuilt one and I know sometimes they aren't rebuilt perfectly - and this one sat for a very long time (multiple years) before I installed it. I'm going to think on it a bit today. At the very least I'm going to pull the master cylinder and give it a good once over. If there are any grand ideas on what to try let me know, I'm open to suggestions.

I'm going to start working through the troubleshooting steps on this page which appears to be an excellent resource:
http://www.classicperform.com/TechBook/ ... shootbleed

[vito]

Yup brakes are not fun. The lines are installed in areas were you can't get a wrench.

Here are the results from the Teflon tape being soaked in brake fluid.





This is after 16 hours-ish. A/C oils would actually eat.

Before you take the MC off, I would hook up some lines to it and place in a bucket and see the flow from there just to verify it is the MC.

[MaineVette]

Thanks Vito, I'm glad I resisted the urge to use teflon tape.

I've played around a bit more this afternoon and have ruled out a couple things. First, with the master cylinder on the car, I removed the brake lines and plugged the holes with bolts. When I tried the pedal it was nice and firm and it didn't sink any lower as I held pressure on it. Then I removed the MC, removed the bolts and actuated the plunger a few times. It gave a few healthy squirts of brake fluid out the side which again seems like a good thing. I did notice a small "fountain" of fluid squirt up from the base of each bowl but I think that's normal.

So, to recap, I've verified my proportioning valve is clear and that the MC can develop and hold pressure. I'm really scratching my head as to why I have a soft pedal. It's hard to believe there was air in the lines after pulling so much fluid through the lines but I guess anything is possible. I'm still perplexed as to why it was so hard to pull fluid through the lines.

I think my next step will be to try my pressure bleeder on the bench (at very low pressure) and see what happens. I would expect brake fluid to come squirting out real quick but I could be wrong.

At the very least I think I'm going to buy some speed bleeders and remove the entire question of pressure bleeder and vacuum bleeder out of the equation. I figure any monkey can push a brake pedal over and over to get the air out and the speed bleeders should make it a one man operation (or at least one man plus a spotter looking for bubbles).

What a pain. I never imagined bleeding the brakes would be so difficult.

To rule out any final gremlins I'm probably going to crack the bleed screw open on one of my calipers and see if I can blow some compressed air through it. I figure the fluid should come gushing out. If it does than the slow flow of fluid must be a MC issue.

And, for the record, this still beats a day at work!