caliper problem

try as I might I cannot get the inboard side to squeeze. I put in new O-rings and stainless pucks and bled as usual. Thoe outboard side moves in and out, the inboard side refuses to fill with fluid. I can see it beginning to move but that is it. Under pressure the disc is actually being shoved inward by the outboard side. Everything is very clean and the passageways are open.

If fully bled, the inner must be full of fluid! I'm wondering if you have one or both pucks in backwards. The open end goes towards the pads of both.

I had a similar problem after replacing the inner piston a little too far into the caliper, I was only getting real pressure on the outer pad, minimal movement on the inner; could you have done this? (&/or maybe not put the outer one in far enough?). The (Mk3) manual states that (on re-assembly) the piston should protrude 5/16 (8mm) beyond the bore.

Iiirc I 'walked' mine out by removing the inner brake pad pad, gently pumping the brake lever & using a piece of wood to fill the disc void (sorry was about 5 years ago!)

Good Luck!

Seems to be a reasonably common problem.
If the inner piston is pushed all the way in then it can blank off the fluid drilling!

If there is a good sized air bubble in passage to inboard piston, it will not be getting same pressure as other piston, so other will move until it meets enough resistence that stuck side gets sufficient pressure to overcome its sticktion. May not get there before you run out of brake lever movement.

Try removing caliper from fork and inverting it to help fluid displace air. Rebleed in upside down position and use a wood block to jam against moving piston while appling pressure to move stuck one.

remove caliper from fork leg but leave the pipe connected. remove the pad on the 'stuck' side and fill the disc slot with a close fitting metal bar (the shank of a suitable spanner will do). Pump the lever and eventually the piston will move. You said you've renewed with stainless pistons. Did they have a slightly raised portion on the fluid face? That is to allow fluid to get round the back of the piston and so make bleeding easier. What was the surface finish of the new pistons like? Should be like as fine as possible, almost mirror like.

the passageway from one puck to the other is small. I would guess the outboard gets the most pressure. I will try the above methods. I have run into the problem of the puck being all the way in the caliper and not filling so I am past that hurdle. Ran out of time yesterday but will try again Monday. I am not an expert on hydraulic brakes but there seems to be a conflict when the connecting passage from right to left is also the bleed hole. Makes it difficult to for the inboard puck to fill with fluid. If the crossover passage fed the forward part pf the rear puck then the fluid would have to travel across the puck to reach the bleed hole, making it easier to bleed. Am I correct or dreaming?

seattle##gs said:

the passageway from one puck to the other is small. I would guess the outboard gets the most pressure. I will try the above methods. I have run into the problem of the puck being all the way in the caliper and not filling so I am past that hurdle. Ran out of time yesterday but will try again Monday. I am not an expert on hydraulic brakes but there seems to be a conflict when the connecting passage from right to left is also the bleed hole. Makes it difficult to for the inboard puck to fill with fluid. If the crossover passage fed the forward part pf the rear puck then the fluid would have to travel across the puck to reach the bleed hole, making it easier to bleed. Am I correct or dreaming?

Click to expand...

The caliper is designed for pre MK3, hold it in the fitted position on the rhs and you will see why this becomes a problem on the MK3, the drillings are not ideal for MK3.
If you have one side full of air and one piston sticks, the pressure exerted on each piston is still the same, but because of the air the system is not operating at correct adequate pressure, ie inefficient. No matter how well machined one piston and caliper bore, one will always move easier than the other due to overcoming friction. At the moment, as one piston experiences pressure and moves, due to the air in the system the volume occupied by the air increases as the piston moves, overall pressure drops and the other piston stays put, the lever moves further to compensate, but you run out of lever movement and the system then has a static pressure.
You need to ensure the system is correctly filled, and then if the piston sticks, polish the periphery off the sticking piston until you can see both move together at a similar rate.
At the moment your system has too much air to exert adequate pressure to move both pistons together.
Pretty basic, but hope this helps in which order you need to check things. There may be nothing wrong with the pistons.

It is hard to block the hole due to the clearances, but if the piston is in the way of the hole, the fluid will struggle to get past the piston and let air back up the fill drilling as you have spotted, this is why it struggles to fill with fluid. It would fill eventually but take some time. Pressure will easily find its way to the chamber.

If I filled the MC then cracked the bleeder (using a hose down to a catch tank) and let it dribble away for a few hours perhaps the fluid would find its way into the inboard cavity and let out the air that's blocking it. Sound like it might work?

Did you clear all the cross drillings with a drill bit? Mine were completely blocked with crystallized material.

If all else fails pump the lever very quickly, then when you have the best pressure, pull it hard back and tie it to the bars and leave it 12 - 24 hours.

Cheers,

cliffa.

As Cliffa states, cable tie the lever to the bars as tight as possible, in addition turn bars to the left so that the m/c is as high as possible & put her on the side stand (you wait til you try a Lockheed
CP2696 - that is a pita on a Mk III )

I just studied an empty caliper again. I see no way for the passages to be open and not to have fluid in both sides if you've bled the air. The bleed port is a drilling directly into the inboard chamber. The fluid enters the outboard chamber goes through a drilling in the outboard chamber that intersects with the drilling from the bleed screw to inboard chamber. Assuming that bleed screw is topmost, the fluid will displace the air - fluid being heavier than air. If the drilling from the bleed screw to the inboard chamber is blocked then it could seem like it is bled when no fluid has entered the inboard chamber - basically only the outboard chamber is bled.

Unlike car brakes, bleeding these takes a lot of pumping, not harder pumping IMHO. You're trying to get air to come out the same passage you're pushing fluid through.

No need to dismount the caliper. If your issue is that the inner piston is too deep simply remove the line from the master, at the caliper, and hit the open hole with compressed air. That has always worked for me and is safe, start with 20 psi and work up, but stop at 50 psi. If the inner piston hasn't moved at 50 psi you have other issues I do suggest that you cover your paint work in the case that there is any blow-back.

Best.

I have run a piece of wire down the crossover passage (my own name for it) and it's clear. Not a trace of crud. Everything is as clean as I can get it. I have run compressed air down the bleed whole and the puck moved. It is not stuck. My theory is that there is very little room behind the puck so the fluid does not want to run in and fill the entire cavity. It would rather come out the bleed hole. Somehow this caliper is more reluctant than usual to fill the inboard side. I will try pumping slowly...my assistant, the owner, was pumping as rapidly as possible.

I either have real good hydraulic luck or just struggle, so if you find something that works lets me know

I must ask: have you tried pressure bleeding from the master down and in the other direction as well and perhaps a
vacuum bleeder pulling the fluid down from the master? Currently wrestling with the same problem one that has never been an issue before. Suspect it is due to the virus!
One problem is that the bleeder screw leaks around the threads if you open it too much. If it doesnt leak it seems to offer a very small hole and not much fluid moves.
A big PLUS 1 on cover all paintwork!

seattle##gs said:

I have run a piece of wire down the crossover passage (my own name for it) and it's clear. Not a trace of crud. Everything is as clean as I can get it. I have run compressed air down the bleed whole and the puck moved. It is not stuck. My theory is that there is very little room behind the puck so the fluid does not want to run in and fill the entire cavity. It would rather come out the bleed hole. Somehow this caliper is more reluctant than usual to fill the inboard side. I will try pumping slowly...my assistant, the owner, was pumping as rapidly as possible.

Click to expand...

As you have read in this thread, if the inner piston is installed too deep it will either preclude bleeding or make it quite frustrating at the least.

You say that the inner piston moves with compressed air; that would indicate that the inner piston is, now, not too deep, good move. The next issue is that the "chamber" behind the inner piston has only one passage for brake fluid to enter, a "dead end" unlike the outer piston/chamber that has a feed and a bleeder; the inner would love a bleeder valve.

Pressure bleeding with any method forces the brake fluid toward the inner chamber, but leaves no way for the air to escape, sort of a one-way street. Cutting to the chase: I use a small syringe and fill the caliper slowly, I use the bleeder valve bore because it is the highest point. Drop by drop, slowly until fluid no longer enters the caliper body, and when you think you have reached this zenith step away and do something else for an hour (YMMV). If you have become so fixated that you can't leave then invite your drinking buddies or your love interest; time always passes quickly in either case.

Once the caliper is really full bleed the line from the master very gently making sure that the reservoir is the highest point. At this point you may be done, if you get a spongy feel let it have the night off, both of you will be good the next morning.

Best.

RoadScholar said:

Cutting to the chase: I use a small syringe and fill the caliper slowly, I use the bleeder valve bore because it is the highest point.

Click to expand...

Adding to your method...

If you were to take a syringe with a needle whose outer diameter is smaller than the bleeder to inner chamber passage and that was long enough to reach the back wall of the inner chamber, you could fill the inner chamber and push the air out easily all at once. This would turn this into a 2-minute job to bleed a Norton caliper! I hope I can find such a syringe.

Today, I figure it takes about 100 light squeezes and 30 minutes all while keeping the master cylinder from running out of fluid and not squirting it all over the place.

I like the ideas with the syringe. Do you know the diameter of the bleed hole? I would like to find a needle that would fit and still allow air to escape. Next problem is finding a needle that will pass brake fluid AND fit down the hole. The problem seems to be poor engineering (or engineering on the cheap). If the crossover passageway had been drilled at the opposite end of the cavity I think it would bleed right away.
It is good to see that others have had the very same problem. I will go to the local veterinary and discuss needles.