Front brake problem

[moizeau]

Just putting the ST back together and took the opportunity to swap the master cylinder to splitter brake hose. Can I bleed it, like hell. The master cylinder was overhauled 4/5 years ago as were the calipers. Prior to this issue the brakes have been outstanding albeit with a slight issue that would cause the calipers to tighten up and the front brake would drag a bit, seemed only to happen in warm weather. Quick 1/4 tun on a bleed nipple and all would return to normal. I had put this down to a poor condition top hose that was restricting the back flow or the brake fluid had got some humidity in it, It seems not.
Spent bloody hours yesterday, standard bleed procedure, then altering the angle of the splitter, centre stand, side stand, change the lock. Gave up and tied the lever in overnight.
Tried again today, no fun. When the lever is pumped a small stream of bubbles come out the hole nearest the lever, RHS one on this photo.

Tried a reverse bleed from bleed nipple back up to the master cylinder. Long bit of clear tube full of brake fluid with my gob on the end. I could get some in then no more. Tried a pop bottle as a squeezy, not much better. Stripped the lot down to check the hoses and splitter for blockages, fine.Blew through each caliper, fine. Then tried the master cylinder, can't blow in reverse (lever not pulled in). Stripped it. The piston and seals still look like new, no leaks, air or fluid. The hole on the rhs is clear the lhs one seems solid. I remember someone, possibly Philippe, mentioning a microscopic return hole? Would that be the 'blocked/solid one on the left? Also as seen in the photo the piston is placed pretty much where it would be when together. The front seal sitting between the 2 'holes'. So that would suggest all brake pressure is applied from the rear seal near the lever.
After cleaning the MC up, did a tongue test. Press the piston forces air out, release the piston and tries to suck your tongue inside. This is with no cover or seal on the MC.
So, should the lhs 'hole' have a hole, if so, how small is it?
Am I correct in thinking the reverse bleed procedure is with the lever out?
Are my gob/lungs or a pop bottle sufficient to force the fluid the wrong way?

All help welcome, cheers

[z2.john]

What fluid are you using? I changed from DOT 4 to DOT 5 a while ago and it took me about 2 hours to bleed.
I was warned that bleeding with DOT 5 takes more time.
My hand was dropping off by time I finished but boy, the sense joy when it finally worked.

[moizeau]

Cheers for the reply, I was hoping to have that sense of joy today....but no. Dot 4, brand new unopened (until earlier)

[warren3200gt]

It's obviously working as it should or you wouldn't have had the small stream of bubbles appearing in the mc. There is obviously still air in the system. Make sure everything is tight so there's no chance of air binging sucked in on lever return the reverse bleed them using a serynge slowly to allow the air to escape from the micro return hole. The other suspect point is the splitter. Unbolt it from the frame and allow it to hang so the highest point is the splitter to mc inlet. They can be bastards for trapped air in the brake light switch and that end of the splitter area.

[moizeau]

Thanks Warren, done all the juggling of the splitter. Is that return hole the lhs one on my photo? Got an idea that my me the problem. The piston forces the fluid down but then sucks it back. It is impossible to get a reverse bleed, The MC won't accept anything in the wrong direction

[Al]

I have one here the same as yours but from the J. It has a microscopic hole in the LHS as viewed in youre picture. The hole looks rusty in the bottom suggesting it has a steel ferrule in it but is probably the exposed top of the steel sleeve,.. but definately a hole. Perhaps 1/4mm in diameter or so!!
AL

[moizeau]

Thanks Al, that's what I was hoping

[warren3200gt]

Pull a bristle from a wire brush and have a probe to ensure the holes in the mc are clear. You obviously need to be able to relief back into the mc or the brakes would stick on.
Where did you see the stream of micro bubbles coming from?

[moizeau]

The rhs hole, not many really, probably the air between the 2 seals.

[warren3200gt]

If you zoom right close on your picture there is a different coloured area central on the LH hole. Looks like it's rusted up. Get probing!

[moizeau]

Yep, I thought that but my eyes are shite. Without the hole on the lhs there would be no way to reverse bleed, It was defying logic. Off to find a needle

[moizeau]

Excellent, thanks chaps, pin got in then a cheap jet file has cleaned it up

[Skid Mark]

Fair play to you using your lungs to reverse bleed! I’d have an aneurism!
You could use a large syringe. You can get large ones for a couple of euro at a veterinary or Agri supply store. I always keep a few. Handy for lots of things. Really good for setting the oil level in forks also with a bit of tubing.

[moizeau]

Done in 5 minutes. Quick pump of the brake lever to expel air, reverse bleed using my pop bottle syringe followed by a convention bleed to get any remaining air out of the calipers.
What a difference a hole makes.

[Philippe]

Hi Pete

I'm glad you solved your brake problem. The diameter of the small LHS hole is 0.35 mm. I use a small drilbit from my modeling tools.
If you want to be sure that your brakes don't tighten up, you have to use the correct brake lever! Most of the pattern new ones have the correct part number but the part that pushes on the master cylinder is a bit too long. That prevents the master cylinder to return fully. If you have that problem you should file a bit of aluminium off the lever. Not too much, a bit at a time. Good luck with it.
GrtZ
Philippe