spongie brakes

[gary coles]

Hi all, got myself a 72 c3 t top in February and have had brake issues since, i have now changed all the callipers, the rear rotors, pads, all flexy lines, booster and master cylinder and the "P" valve, I have bleed the brakes the old way , power bleed them used massive amounts of dot fluid with no success, the pedal is good and stiff until you start the engine then no pressure goes straight to the floor. I have tried everything. Anyone have anymore ideas please get back to me. Thanks

 

[Daytona Vette]

Master Cylinder Issue I suspect, if the pedal is firm and then falls away

 

[gary coles]

Thanks,
Both new bench bleed the master cylinder then set the new booster to m cylinder with the setting tool provided

 

[Forrest Gump]

Could still be air in the system. You could try the old trick of holding down the pedal with a length of timber wedged against the seat and leaving it overnight. Also jack-up the rear of the car so that the master cylinder is slightly tipped forward - that will help air escape from the MC. When the car is on the ground, the MC is inclined the other way and air can get trapped. Put a spirit level on top and you will see how much it leans back in normal position.

 

[gary coles]

Thanks for that, I am working on the car now , driving me mad, I will try that sounds good thank you

 

[Stingray]

Master Cylinder Issue I suspect, if the pedal is firm and then falls away

Agreed. How easy to just get and fit a new one?

 

[gary coles]

This is my 3rd master cylinder and the results are exactly the same, each has been bench bleed exactly how it’s described on the manufactures instructions and YouTube, so the last master cylinder I purchased was a master cylinder/ booster package so I am stumped, I think it’s air trapping somewhere but still open to suggestions

 

[Oneball]

Being able to pump the pedal firm with the engine off and then being able to press the pedal down with it running is just a function of lack of vacuum to the servo and not a fault.

I reckon you’ve got a leak in there somewhere. Check everything is tight including stuff you’ve not changed and check lines for corrosion

After that maybe Try bleeding with the engine running. Push the pistons back. Lift the front end. Check the switch in the p valve is centred. Tilt the front calipers so the bleed is at the top. Check there is a small gap between servo Rod and mc piston.

 

[Stingray]

the last master cylinder I purchased was a master cylinder/ booster package so I am stumped

Hmmm, me too. That sounds strange.

Clutching at straws here, but might there be a leak in the pipework delivering vacuum to the booster? The internet suggests, "One quick test for leakage, is to turn the engine off and press the brake pedal. If the pedal still has one or two assisted applications before getting hard to press, likely no leak exists."

 

[Forrest Gump]

Servo is working well I’d say.….it’s pulling the brake pedal down further than you can push it when the engine is off - without it’s assistance.
The extra pedal travel is either fluid leaking out or air in the system compressing.

 

[gary coles]

I have checked everywhere for leaks putting cardboard under the car over night and blue roll around all joints and bone dry, so like you say it really has to be trapped air , just can’t think what else it could be, but what’s the best to get air out, there’s definitely no air bubbles in the fluid when bleeding

 

[Derek Nicol]

Sounds familiar.. ish..
I recently lazily booked my C3 into a garage to have a brake fluid change because i know what a pain it can be sometimes.
Turns out the master cylinder was on its last legs so they replaced that but they just couldnt get a firm pedal to the point where they were close to giving up.
Now i hope ive understood the story correctly but what i was told was that it was only when they got four mechanics and put a mechanic with a reverse power bleeder on all four calipers simultaneously reverse bleeding from all four corners that they eventually managed to get it clear of air.

 

[teamzr1]

I have checked everywhere for leaks putting cardboard under the car over night and blue roll around all joints and bone dry, so like you say it really has to be trapped air , just can’t think what else it could be, but what’s the best to get air out, there’s definitely no air bubbles in the fluid when bleeding

Have you tried using a pressure bleeder like this ?
Pressure Bleeder


Or also check I suspect vacuum leak using a smoker you could buy, borrow, or can make one

 

[gary coles]

yes i have got one very similar to the one shown

 

[Emc]

Gary feel free to call me on 01283 219420 I'm sure I can help

 

[Forrest Gump]

Any luck solving this problem?….just so we know what worked for future reference.

 

[Piginvet]

Just my penniesworth of experience....mine had similar brake problems, went through gallons of fluid....guy before me owning, never cured it fully despite being a garage!
However, one day by chance i found hairline crack in the rear brass control balance valve. Swapped for new one, bled all calipers again (banging each one with rubber mallet) before locking off and bingo...
Wirth a look at if all else has been done which sounds like it has!