Air getting into brake system

[ianc9]

My '68 LHD S1.5 2+2 appears to be getting air into the brake system. Its got the remote servo and uprated front calipers.

I bleed the brakes (done at 5 times now) and get a half decent pedal - Not perfect, but OK. I'm not sure what "Perfect" feels like for a '68 E-Type though since this is the only one I've driven :)
Over a day of driving the pedal reverts to needing a pump to be confident.
Bleed them again and you're back to having an OK pedal.
The pedal needs to be pumped even when testing it stationary - So I think that eliminates pads getting pushed back due to disc distortion etc.
Also needs to be pumped if vacuum system is disconnected.

I pulled the master cylinder out and rebuilt it using the kit which includes pretty much everything except the reaction valve diaphragm. It looked pristine when I did the strip down (I think it was replaced by the PO about 3 years ago). I went ahead and replaced all the seals, plastics and piston anyway just to be sure.
No improvement :(

There's also a small notchiness at a single point in the pedal movement which is more evident when there is air in the system.

Has anyone else experienced this or have any suggestions on where to look next?

[Jack the lad]

When you re-bleed the system, do you get any air out or does it seem to be good fluid only. Also how are you bleeding it, with a traditional pipe and jar or do you have a pressure bleeder. Did you start at the bleed valve furthest from the servo and work your way back towards it? If you suspect a particular wheel being leaky fit a pipe clamp (cheap on ebay, get a Sykes Pickavant if possible)to the flexible pipe to isolate the particular calliper and see if the pedal has improved, I know its tricky on the rear as only the one flexible hose, but it will give you an indication where to look (front or rear).

[ianc9]

When you re-bleed the system, do you get any air out or does it seem to be good fluid only. Also how are you bleeding it, with a traditional pipe and jar or do you have a pressure bleeder. Did you start at the bleed valve furthest from the servo and work your way back towards it? If you suspect a particular wheel being leaky fit a pipe clamp (cheap on ebay, get a Sykes Pickavant if possible)to the flexible pipe to isolate the particular calliper and see if the pedal has improved, I know its tricky on the rear as only the one flexible hose, but it will give you an indication where to look (front or rear).

Yes I do get some air out.
I've tried:
- Traditional pedal pumping with friend opening and closing nipple
- Pressure system (made a complete mess)
- Syringe
- Vacuum based system
All gave an ok result
and I started at back
All flexible pipes are braided so I don't think I can clamp them without damaging them

[mgcjag]

You should have slight movement of the brake pedal to start with then becoming rock hard.....to bleed as above start at rear....with tube attached open nipple press pedal down lock nipple release pedal.....repeat....don't ever let pedal up if nipple is open as it can suck in air..Steve

[ianc9]

You should have slight movement of the brake pedal to start with then becoming rock hard.....to bleed as above start at rear....with tube attached open nipple press pedal down lock nipple release pedal.....repeat....don't ever let pedal up if nipple is open as it can suck in air..Steve

I saw the "Slight pedal movement" comment elsewhere on the forum and checked. It feels like I have about 5mm movement.

I'm getting a pretty good result with the bleeding. It just appears to pull air into the circuits after a short time.

[mgcjag]

Hi Ian.. you don't have to quote everything in your reply it just takes up valuable server space......cheers..Steve

[mgcjag]

Bleed again one press at a time locking nipple in-between.. if you start "pumping" you just move any air that's still in the system backwards and forwards without pushing it out.....

[ianc9]

Sorry I was unclear in my earlier description. When we did it pumping the pedal we did it exactly as you've described.
Pedal down
Open to release, close to seal
Pedal up.
Repeat

[mgcjag]

Not quite......should be open nipple..pedal down...close nipple....release pedal.....repeat...Steve

[ianc9]

Agreed. Not identical. I have done the way I describe to avoid the, admittedly small, chance that there is vacuum in the line and opening the nipple might draw air in. It also allows you to pump the pedal before opening when there is a lot of air in it. I can't think of a downside to doing it this way.

In any event - the problem appears to be air ingress after the bleeding has been successfully completed.

[Jack the lad]

It is possible that you have a small amount of air trapped within your system that you are not getting out, it could be in the pipes close to the master cylinder or the brake booster. Try opening momentarily (cracking open) each of the metal pipe connectors on the master cylinder and the booster whilst your friend holds down a firm brake pedal. Use a clean cloth to prevent the fluid going where it is not welcome, when you have closed off the open pipe have him quickly release the pedal so it will fly back quickly. This works well on systems that are troublesome.

[DWW]

It stands to logic that if you have air ingress you should have fluid outgress. So if your piping and connections are dry all over and you have no signs of leaks under the car, it could be a trapped air bubble, as mentioned, especially since you are not losing any brake fluid. Check that all the bleed nipples are air tight, especially the rear.

[ianc9]

I agree with both of these points and can confirm no detectable fluid loss but it doesn't explain why the pedal is good for a short while and then get bad again.

I am going to try bleeding as described above one more time (with feeling ) to see what happens

Thanks all