Air in brake line

[CliveChafer]

I had my mechanic replace the two wheel cylinders on the front driver's side wheel of my 68 Minor 1000. When I got it back the brakes stopped me nicely, but the pedal was nearly going to the floor. I took it back and they bled the system again, but to no avail. I checked the adjustments on the cams on each wheel, and they are all nice and close. The pedal definitely pumps up, however. So I tried bleeding just the wheel where the cylinders had been replaced, and lo and behold, bubbles. LOTS of bubbles. In fact, a never-ending stream of bubbles! I've put enough fluid through from the master cylinder now that it has all been replaced. So not only is there clearly air in the system, it seems to be getting more air in it as I bleed it. Any ideas? Master cylinder problem? All suggestions gratefully received.

[Myrtles Man]

"So I tried bleeding just the wheel where the cylinders had been replaced, and lo and behold, bubbles. LOTS of bubbles. In fact, a never-ending stream of bubbles!"

Sounds to me like you've probably got air finding its way in past the bleed-screw threads. Are you loosening the nipple by just a quarter turn or 'giving it plenty'?

[CliveChafer]

I'm loosening it by no more than a quarter turn. The fluid is flowing through very well at that position, so no need to open it further. But you're right, it does sound like it's getting in at the very end of the line, past the threads on the nipple. That's the first thing the mechanic said at East Sussex Minors too. But I would think if that was the case that there wouldn't actually be any air in the cylinder or the system further back, so I should be able just to close up the nipple at the end of the stroke and not have any air get back up the line. It's weird.

[Peted7202]

Why were the wheel cylinders replaced?

[philthehill]

Have you got the right ends on the metal brake pipes to suit the cylinders?
The pipe ends could be either concave or convex. If they are not right they can suck in air. It is not the threads that seal it is the pipe ends which must sit correctly on the seat inside the cylinder to be effective.

[Peted7202]

]Here is a photo of balance pipes supplied recently. The threads are correct 3/8 UNF but the flaring is incorrect. The original bridging pipe on the right is single flare. The 2 supplied ones are double flare and are unlikely to seal.

[CliveChafer]

That's fascinating. I will have to look at the end of the brake line to see what they fitted. They replaced one short piece of line to the brake cylinder from the junction below the radiator. The wheel cylinders were replaced because there were signs of leakage in the drum (it was damp).

[olonas]

After reading and seeing that picture I thought it wise to check the new/spare pipes I have. Yes, they have a single flare at each end.
It's something that I hadn't even thought to check.
It's usually never safe to assume anything, but I have to assume those fitted to my Minor are correct. They were replaced some years ago along with the hydraulic cylinders.

[niftyrodman]

Sometimes when a master cylinder is in need of replacement, and the hydraulic cups are worn or you have a pitted barrel it will suck air in on brake pedal return, in from the exposed front. No amount of bleeding will cure that problem.

[oliver90owner]

Sometimes when a master cylinder is in need of replacement, and the hydraulic cups are worn or you have a pitted barrel it will suck air in on brake pedal return, in from the exposed front. No amount of bleeding will cure that problem.

BUT…. very unlikely to occur exactly at the same time as other (suspect) work has been carried out remote from the master cylinder?

[olonas]

Would a "pressure" bleed help? I adapted an old Lockheed master cylinder cap to use with my, initially EEzibleed, then further adapted to use on my pump up pressure bleeder.

[niftyrodman]

Yes I agree that may be the case ie was OK before I touched it. So buy 2 pairs of hose pliers and clamp off the front brake hoses and see whether your pedal pressure stays, then do the same with the rear. It can hardly be a brake pipe fitting or the fluid would leak under pressure. Sometimes when bleeding brakes it pays to let all nipples drip and keep an eye on the master level. Self bleeding that way works.

[niftyrodman]

Yes I agree that may be the case ie was OK before I touched it. So buy 2 pairs of hose pliers and clamp off the front brake hoses and see whether your pedal pressure stays, then do the same with the rear. It can hardly be a brake pipe fitting or the fluid would leak under pressure. Sometimes when bleeding brakes it pays to let all nipples drip and keep an eye on the master level. Self bleeding that way works.

[niftyrodman]

On the question of brake bleeding, MMs are so simple, and pressure bleeding is simply a waste of time.The person posting "air in the system" may be making the wrong diagnosis, I mean the only way in my 50 year experience when air seems to come from nowhere is the maater cylinder

[olonas]

"On the question of brake bleeding, MMs are so simple, and pressure bleeding is simply a waste of time."
Why? It makes it even simpler.
I have never found there's enough "head" to just open a nipple and gravity bleed.
If it is air being drawn in to the master cylinder why was the pedal o.k., presumably, prior to the wheel cylinder replacement?