Brake problem

Post by **Heuer** » Wed Jul 16, 2014 5:21 pm

I agree with Mark, sounds like you boiled the fluid after a spirited mountain run. Could be caused by a seized calliper piston, old brake fluid, moisture in the fluid and/or not allowing the brakes to cool between application. You will need to change the fluid now anyway but check none of the pads are binding by jacking up the car, applying/releasing the brakes and rotating the wheels in turn. Also if you have a 4.2 make sure the servo shield is in place (it was a retro fit on some cars). Are your brakes standard?

Maikel · Post by **Maikel** » Wed Jul 16, 2014 6:10 pm

Hello Mark,

thank you for your answer, but I do not believe that brake flid can evaporate.

I am tring to narrow my case.
Lets assume there is a leakage, so why could I not pump and get pressure.

Lets assume there is still a leakage, why can I brake now when the engine is off and also after starting it works all fine.

What makes the pedal come back? Is that a spring or underpressure in the master Cylinder?

For me there are two issues
1. No resistance at the pedal
2. no returning pedal

Thanks in advance for your comments
Maikel

Mark Gordon · Post by **Mark Gordon** » Wed Jul 16, 2014 7:28 pm

Like all liquids, brake fluid does have a boiling point. It is not lost from the system and once the source of the heat is removed or cools down, the brake fluid vapor recondenses into a liquid and you again have fairly normal brake function. There really is nothing else that can cause you to suddenly lose all brake pressure and then after the car sits for a while and things cool down, for your brake function to return. Something is causing your brakes to drag and even a slight drag that might not be very noticeable will cause the caliper to heat up and then eventually boil the fluid. Your fluid is not evaporating and being lost, otherwise your brakes would continue to fail.

christopher storey · Post by **christopher storey** » Thu Jul 17, 2014 7:34 am

And just to add to this, normal brake fluid is hygroscopic and condensation in the feed cylinders eventually finds its way into the system. This lowers the overall boiling point even further. This applies even with silicone fluid - there the water does not mix with the fluid but creates pools at low points of the system

David Oslo · Post by **David Oslo** » Sun Nov 23, 2014 4:31 pm

It sounds like one of the seals has failed on the tandem slave cylinder (i.e. the piston within the master cylinder), such that pedal effort is not giving equal pressure to both brake circuits, but instead the little piston has bottomed out (due to failed seal) and is only working one of the brake circuits.

Hence you will have felt the pedal "dissappear" as the seal fail and everything moved an extra amount. But you will still have braking, but somewhat reduced (halved). And you will still be able to feel a "hard" pedal.

If you look at the cross-section drawing in the workshop manual then you can start to see how this occurs.

You may also notice that one of your brake reservoirs has dropped a bit in level.

How do I know ... well Maikel's driving description matches exactly what happened to me this summer ... including handbrake braking and low gear whilst slowly not panicking ... waited a bit and the pedal was back again.

Floats · Post by **Floats** » Sun Nov 15, 2015 6:05 am

Hi Guys,
I am having difficulty sorting the air regulator valve ARV on the master cylinder.
Christopher has given a lot of good advise on this forum, but I am still battling.
My MC was profecionally rebuilt and the ARV seal and rubbers are good, however the little shuttle sticks in the open position. The shaft facing rearwards as well as the other end are nice and smooth, the rubber seal on the shuttle is new and a nice tight fit. It was suggested earlier that the spring should be streached a little to alleviate the friction.
My question, are there more than one spring supposed to be inside the ARV. There is no way that the little spring in the ARV air filter cap can be stretched enough to push the shuttle back. A stronger spring will pop the air filter cap. Should be an extra spring inside the ARV to push the diaphram back on brake release?
Regards
Chris
Cape Town

Post by **mgcjag** » Sun Nov 15, 2015 10:05 am

Hi Chris.....there is onlythe one small spring....if you look in the knowledge base section there are service manuals that show diagrams of this in detail.....page L.X.s.8.. just becaus you have had the MC rebuilt dosent mean that its been done correct....you already know that its stuck in the out position...to free this someone here recently had the same problem, pulled out the tiny piston and with some very fine abrasive paper rolled to a tube shape smoothed out the hole it sits in, this solved his problem.....i have had the same piston sticking and solved it by turning around part no28 piston operating lever it has a rounded edge one side and a sharper edge the other ....hope this helps . Steve

Floats · Post by **Floats** » Sun Nov 15, 2015 3:53 pm

Hi Steve,
TKS, I had a look at the diagrams and you are correct. There are no springs inside except for the little one in the white air filter dust cap.
The tiny plunger is NOT stuck in the MC snout. However the new seals holds the plunger in it's tube and the tiny spring in the air cleaner cap is not strong enough to push it back when the brakes are released.
It was suggested somewhere on this forum to stretch the little spring in order to overcome the tention holding the plunger in the extended position. A stronger spring in the cap could cause the cap to dislodge and fall off. My idea is to insert a spring inside the ARV to assist with the retraction of the plunger rather than running the risk of loosing the dust cap. My spring will go between the diaphragm and the white plastic DOME of the ARV, NOT IN THE OUTER DUST CAP
Any ideas Gents?
Regards
Chris

Floats · Post by **Floats** » Sun Nov 15, 2015 4:45 pm

Steve,
I've just tried it and seems to be working fine. Unfortunately, with the brakes applied the engine is breathing through the ARV causing the engine to run lean. On brake release the plunger moves back, as it should, the ARV shuts off the vacuum and the engine recovers and runs normally.
The engine should not breathe through the ARV. The suction should cause a vacuum in the ARV and the booster/servo. The valve of the ARV should then close, maintaining the vacuum without allowing the motor to continuously breath through the ARV.
Any suggestions?
Regards
Chris

Post by **mgcjag** » Sun Nov 15, 2015 5:03 pm

Hi Chris.....read the previous pages to diagram of the MC....it has a diagram of the vacume system for the brakes..and a writen explenation.....something wrong in the ARV i epect.. Steve

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