After much debate on this forum and elsewhere, I've accepted the fact that some prior owner upgraded the brake fluid to DOT 5.

I went to flush/bleed the system today and ran into a problem. Because of the DOT 5 I had decided to not use the Motive pressure bleader I bought and instead had my son in the car. Per a post by Duke I started with the left front, shortest route, to get rid of the old fluid in the MC. I had turkey basted the MC out and filled it with new DOT 5 prior to doing this. After a few goes, the brake system refuses to pressure up. The reason I was doing the flush in the first place was my pressure differential light (brake light) was coming on.

Am I missing something or has my MC failed?

Thanks,
Bruce

p.s. I've read the archives three-times over.

 

I thought I read somewhere not to agitate the DOT 5 because it will entrap air, which will cause spongy pedal. Not sure if this is accurate or not.

 

You are correct. DOT 5 should be poured very slowly, and only bled after all of the air bubbles have dissapated. Also, never shake a container of DOT 5 and expect to use it within a day!

However, since he has a brake light that indicates a problem in the first place, think the system needs to be fixed first. Why is the light coming on (indicates an imbalance in the system normally caused by a leak)?

Also, I have pressure bled DOT 5 with good results. Just have to use low pressure (10 psi or so) and therefore low flow rates.

Plasticman

 

18 years of dot5 and no problems, but I bleed with a cheap MightyVac at the bleeders.

 

I understand about the air bubbles in DOT 5 fluid and I've chosen not to use a pressure bleeder even though I have one. I'm wondering what kind of issues would cause me not to get any pressure via the brake pedal. Is my MC bad, the pressure regulator or something else. I do not have power brakes nor has any work been done on the car recently. This problem seemed to come on after a particularly hard stop a while back.

 

Willowdog,
Did you drain your system completely, or drain the MC dry at any time? I started my 69' with out bench bleeding and it 4-5 hours of pumping and still no fliud. So I bought a mighty-Vac, and it still took 2-3 hours.

 

Willowdog,
I had the same problem about three years ago with bleeding Dot5. Believe it or not, pumping the pedal to bleed the brakes agitates the Dot5 enough to cause air bubbles in the system. I had to use the power bleeder to get a pedal. If you decide to use your power bleeder, use very low pressure as Plasticman suggests.
Dave

 

If you started at the front of the car to bleed and for no apparent reason the the fluid flow stopped, go to either rear and open a bleeder screw after you pump the pedal. The fluid should flow out the back. It may quit, if it does, go back to the front. While you're bleeding the brakes, look for calipers weeping, seeping or leaking. Check along the main front/rear brake for leaks. If you see nothing, your master cylinder is probably bypassing internally and needs rebuilt/replaced.

 

Plasticman
mgsouthard
Stingrayl76
MikeM

 

If it were me in your situation from what I have read. I would pull the master cylinder and put it in a vise and bench bleed it again. Check the back of it where the plunger is for leaks while your at it. If no leaks and you bench bleed ok. Put the MC back on the car, put a quart of Dot5 into your Motive bleeder, connect it to your MC and start bleeding. On a Corvette always start at the Driver rear inner then outer, then over to the Passenger side rear inner and outer, then to the passenger front, then to the Driver front. Go around once more just to be safe. With the Motive bleeder and a good MC you should be done in no more then 20 minutes. I gave up 2 years ago bleeding manually with Dot5.

 

Keep in mind by manually forcing the brake pedal to the floor there is a risk of ruining the MC piston seals when they are forced over sludge buildup in the MC-bore. If you must use the manual method try to avoid pushing the brake pedal all the way to floor. If you are in doubt regarding the state of the MC remove it from the car and inspect the rear of it looking for a leak.

 

Keep in mind by manually forcing the brake pedal to the floor there is a risk of ruining the MC piston seals when they are forced over sludge buildup in the MC-bore. If you must use the manual method try to avoid pushing the brake pedal all the way to floor. If you are in doubt regarding the state of the MC remove it from the car and inspect the rear of it looking for a leak.

 

1966 non power brakes. New Bendix MC bench primed and tested. Complete flush of system and replace with Prestone DOT 3. All calipers rebuilt w/ S/S sleeves. and no leaks. System bled with GM style bleeder, pressurising the MC. Peddle is high and hard in the garage.
PROBLEM: After an hour or so on the road the peddle stroke gets dangerously long and spongy. Bleed brakes again and find no air in system...I'm going crazy.....HELP PLEASE
Thanks, hgvracer

 

You could have a brand New (Defective) Bendix Master Cylinder. I have seen this before. If there are no leaks and no air in the system I can't think of anything else. Question for you. What did you mean when you said the system was bled with a GM style bleeder? Is it a Pressure bleeder similiar to the Motive bleeder alot of the members here are using? Just curious.

 

If it's bled properly, and there are no leaks, then it has to be a MC gone bad.

Over time, as the car is driven, the MC piston travel settles in at a "sweet spot" where there is a maximum travel (+/-). Just beyond this spot, there will be a buildup of sludge (and rust if DOT3 is used). This forms a ridge in the MC. When you bleed the system, there is insufficient (or no) back-pressure to stop the piston/seal from going past the ridge. When this happens, old seals will fall apart at the edge and leak.

It's very common for MC's to fail during the bleeding process because of this.

Over the past 40 years of doing brake jobs, all I had to do was look inside master cylinders prior to honing & rebuilding them to see the how this all occurs. I also looked at the seals under a microscope to see just how freyed the cups were.

New and/or rebuilt MC's right off the shelf can have the same problem.... they rust internally while sitting on the shelf. When they are bled, the rust chews up the seals.

I disassemble all replacement MC's and inspect them internally prior to installation.

When I'm doing a brake job, I inform the owner that they may have to cough up the change for MC work... even if it's working okay when the job starts.

Also... brake hose ballooning is a possibility... replace them if they are old.

 

If you are losing your brakes after an hour of driving, check your rotors for runout with a dial indicator. Should be no more than .008. Better if under .005. If you have runout, try shimming the rotor with shim stock or moving the rotor on the hub or using O-ring seals on the brake pistons. O-rings in the pistons work to a degree because they are more forgiving in allowing air to be pumped back into the brake system. Runout could also be caused by a warped rotor or a bent spindle in the rear assembly. It really does not take much. Jerry

 

Just my two cents worth, I had a 75 with the aforementioned problems and finally gravity bled the system with great success.
Larry

 

The bleeder is a 3/8 " inch aluminum cap sealed by an O ring set in a groove in the cap. Its held down tight to the MC by a 1/4" dia. bail running under the MC tightened by two 1/4" x 20 wingnuts. The cap has a male 1/4 "air hose connector threaded into it and a 3/4" NPT plug for refilling the MC. Procedure is to fill the MC, put on the bleeder cap, connect the air hose with 10-15 psi over the fluid. Start at the left rear inner bleeder connected by clear plastic hose to a bubble jar. Bleed till no air in hose or bubbles rising in the jar. Refill MC when fluid in bubble jar rises to calibrated mark to avoid running the MC dry.
IF ANYONE THINKS THIS IS WRONG, PLEASE TELL ME SO.
This is the second new MC. The first was my original MC after being sent to Corvette Central for a $250.00 sleeve job.
I'm losing my mind and I'm going to quit Corvettes and go up in the mountains and raise goats.
Thanks, hgvracer

 

When you say "gravity bleed" what do you mean? Hooking up something like an IV drip and openning up all the bleeder screws at once ????

 

I don't understand the part about o rings on the pistons?? I'll check the run out tonight. Rather than trying to shim the rotor, wouldn't it be better to turn them in a brake lathe ???

Thanks for your help