Two pump brake pedal

[basilruler1234]

Hello, I am hoping for a little advice

I have a two pump brake pedal, first pump goes low and then on the second pump the pedal is at the correct height and hard, and whilst held there it doesn't sag.

I have bled the system extensively, no air.

Brake cylinders are good.

No loss of fluid from the reservoirs.

I have performed the test to exhaust the vacuum tank by repeatedly pressing the pedal, it becomes hard and high, foot on the pedal, start the engine, pedal then drops slightly as expected as the servo assists.

I have ordered a seal repair kit for both the master cylinder and servo, with the above information, which do you think is the likely culprit.

thankyou James.
Series 1 4.2

[mgcjag]

Air in the system......old school way to bleed is the way to go.....open nipple...press pedal down and hold it down and close nipple....either use a helper of a length of wood to hold pedal down....release pedal......repeat....do not let the pedal up if a nipple is open...so no pumping....Steve

[basilruler1234]

Yes, this is how I've done it, I've bled the system this way several times, I don't believe there is any air in the system.

[Tommd]

Air in the system is the first concern.
More details. Does it do it even sitting still without the car moving? How long must one wait from the two pumps that bring the pedal up until the pedal now drops low?
If not air I would suspect a caliper piston is retracting too much. If the problem only occurs while the car is in motion, I would check for a warped rotor that is pushing the pad piston back too far. If it happens without the car in motion, I would check for a caliper piston that is retracting too far due to seals or a messed up retract pin system in the caliper.
Remote possibility a weak rubber hose that expands partially on pressure.
Tom

[basilruler1234]

Yes the pedal is the same moving or not and the pedal drops again almost immediately after the pedal is released.
Discs are true, dti check, and checked pad retraction with feeler gauges

[John37]

A sticking brake pad or caliper piston? The first push flexes the disc to meet the non moving pad.

[40GT]

You have original calipers?

4 pot, different story.

[madjack4]

Clamp all the flexy brake pipes u should have a solid pedal release one at a time when u release the one that has a problem it will move down a lot more than the rest bleed that one with the other pipes clamped and check that calliper make Shure one piston is not seized and flexing the disc

[abowie]

viewtopic.php?t=6273

My guess is air as well. Try pressure bleeding as above.

[basilruler1234]

I've tried to pressure bleed as suggested, the back went fine, but when I came to the front, I can't get any fluid at all through now. I had bled them successfully a few days ago.

Fluid is getting from the master cylinder line to the servo, but I can't get any fluid out of the slave cylinder line that feeds the front brakes.

Any thoughts?

[Series1 Stu]

Any thoughts?

Yes, the shuttle valve in the slave cylinder is pushed forward, stopping the fluid from flowing to the front brakes. I chased this fault on my 420 for ages, must have used 3 litres of brake fluid before I reverse bled the front brakes, which must have pushed the shuttle back to allow the fluid to flow properly.

Good luck.

Regards

[mgcjag]

Just for clarity have you worked or renewed anything in the braking system prior to haveing this 2 pump issue....Steve.....

[basilruler1234]

Success!

Pressure bleeding led me to believe that the left reservoir fed the rear circuit. In fact it feeds the front on my car, but because the slave cylinder was allowing fluid to pass between circuits I was able to bleed the back circuit with the wrong reservoir pressurised. But could not get any fluid to the front, probably due to the shuttle valve sticking.

I replaced the servo wholesale with a new unit, and was then confused why I still couldn't bleed the front despite removing the stuck shuttle valve issue, by having fitted a new servo unit.

After trying to fruitlessly get fluid through to the front calipers, I went to the other reservoir to see if there was some sort of air lock I could dislodge by pumping fluid through the other circuit, to my surprise the fluid flowed freely out of the front. I then realised my original error in which reservoir fed which circuit.

All in all very frustrating. But a rebuilt master cylinder and new booster later I have a brake pedal that could be sponsored by Viagra.

[abowie]

Good outcome. The early 4.2s had the front and rear circuits reversed in comparison to all the later cars.