17 replies to this topic

[JonnyAlpha]

Hi, I have a second hand (nearly new) set of 7.5 Disc Calipers, ready for a Subframe and Engine replacement. I have just checked the pistons using compressed air. With some wood between the pistons, one caliper seems to work fine, i.e. both pistons move in and compress the wood. One the other caliper, one of the pistons (and it's always one isn't it), was stuck. I tried locking the moving piston in but still the second one would not move. Then I removed the bleed nipple and blew air in expecting it to come out of the nipple hole and it did not. I then blew air into the bleed nipple hole and hey presto the piston that was not moving slid out lovely. Now when I put the nipple back in or out only one piston moves (the one that moved first) the one on the opposite side of the bleed nipple does not want to operate? So the piston is not stuck it just won't move when air is forced into the main brake hose hole, but will if I force air in through the bleed nipple hole? Could it be theres no brake fluid in one caliper but there is in the other?

Edited by JonnyAlpha, 03 September 2021 - 01:38 PM.

[KTS]

not sure i follow that completely, but could it be the drillings that supply fluid between the two halves of the caliper are misaligned / blocked ?

[GraemeC]

Nearly new indicates they've previously been used?

It does sound as though something is blocking the drilling between the bleed port and the fluid chamber behind the inboard piston).  You may be able to fill them with brake cleaner and see if that dissolves/shifts whatever it is, but chances are it will still be in there.  Or if you're really lucky poke some welding wire down the hole in the bottom of the bleed port.

 

Personally I'd be splitting the halves an investigating - I'd advise having a new seal kit and the seal between the two halves on standby though.

 

I may have half a seal kit (which is all you really need as the inboard piston sounds fine)

Edited by GraemeC, 03 September 2021 - 03:25 PM.

[viz139]

I would have thought that blowing air through both directions would have dislodged any dirt blocking the link between the two halves. I would suspect that somebody has put two wrong halves together. Split and investigate.

[Tornado99]

A vintage motorbike restoration video I saw recently showed a way to unseize brake caliper pistons using a grease gun on the bleed screw and forcing grease in under pressure. Of course it needed thorough cleaning out later. 

 

Note if you've freed up both pistons and are now only seeing one side moving under equal loads from air compressor, this is not unsuual as the piston with least restriction will be only one moving until it encounters a greater restriction than the other one. This is a "path of least resistance" effect. Can you hold the moving one using a C-clamp or such while applying your air pressure? other side should then move. 

Edited by Tornado99, 03 September 2021 - 04:44 PM.

[JonnyAlpha]

Nearly new indicates they've previously been used?
It does sound as though something is blocking the drilling between the bleed port and the fluid chamber behind the inboard piston). You may be able to fill them with brake cleaner and see if that dissolves/shifts whatever it is, but chances are it will still be in there. Or if you're really lucky poke some welding wire down the hole in the bottom of the bleed port.

Personally I'd be splitting the halves an investigating - I'd advise having a new seal kit and the seal between the two halves on standby though.

I may have half a seal kit (which is all you really need as the inboard piston sounds fine)

I’ll take you up on that.

[JonnyAlpha]

A vintage motorbike restoration video I saw recently showed a way to unseize brake caliper pistons using a grease gun on the bleed screw and forcing grease in under pressure. Of course it needed thorough cleaning out later.

Note if you've freed up both pistons and are now only seeing one side moving under equal loads from air compressor, this is not unsuual as the piston with least restriction will be only one moving until it encounters a greater restriction than the other one. This is a "path of least resistance" effect. Can you hold the moving one using a C-clamp or such while applying your air pressure? other side should then move.

I’ve trapped the moving piston, but blowing air in the brake line hole does nothing. Blowing ar in the bleed nipple hole however and the non moving piston moves fine. All piston are nice an shiny and seals look all good.

[weef]

This maybe not applicable if you only want to do this once, but is a good fix to remove stuck brake caliper/wheel cylinder pistons. This was something that used to be encountered quite often when a vehicle was laid up or the brakes were only used lightly and I found that wheel cylinders were more prone than calipers to seizing.

Using a new grease gun fitted with a brake flexible hose, I was lucky to find one that fitted without having to use an adapter, rig up a short piece of brake tubing with the correct tube nuts to fit the caliper and attach same. Supporting the grease gun, with the lever down, pour  brake fluid into it . Now bleed the system then use the gun to pump the stuck piston out, trapping the one that moves so the force is applied to the stuck piston.

In your case remove the piston that does not move, pump the gun, and look for fluid entering the chamber. This may be enough to clear any blockage. If no fluid is seen,  strip the caliper for investigation. 

ALWAYS wear eye protection when using this method. 

[panky]

I made this up to pop the wheel cylinders on my Minor traveller, something similar would work on calipers. I used red rubber grease in the gun.

 

[viz139]

The way the calipers are made is after casting the pistons are machined out. For the fluid tubes are drilled from

1. piston 1 to the hose connection,

2. from split joint to piston 1 

3.from split joint to bleed nipple and

4. from bleed nipple to piston 2. 

You have confirmed that both pistons are free and that tubes 1 and 4 are clear. If you split the caliper you will have full access to clean out tubes 2 and 3 with a small drill bit and confirm that you have the two correct halves. If you have no leaks so far and the units are fairly new I would be happy to reuse all seals.

[Homersimpson]

If you blow air into the brake line connection does it come out of the bleed hole?  If not then you need to strip the calliper to find out why.

 

Bear in mind that compressed air pressure is very low in comparions to the hydraulic pressure that the brakes apply so compressed air won't always unstick a piston.

 

I've just stripped a pair for a jaguar 420 and got them out as mentioned above by getting a male brake fitting, drilling and tapping it for grease gun fitting and pumping them full of grease, of course they now need to be totally stripped to get all the grease out!

[Homersimpson]

Also to add that you might need the piston on the side that the hose connects to slightly out before air/fluid would flow to the other one, when fully retracted its possible that it might block the hole to the other side due to tollerances in the calliper so check that as well.

[JonnyAlpha]

The way the calipers are made is after casting the pistons are machined out. For the fluid tubes are drilled from

1. piston 1 to the hose connection,

2. from split joint to piston 1 

3.from split joint to bleed nipple and

4. from bleed nipple to piston 2. 

You have confirmed that both pistons are free and that tubes 1 and 4 are clear. If you split the caliper you will have full access to clean out tubes 2 and 3 with a small drill bit and confirm that you have the two correct halves. If you have no leaks so far and the units are fairly new I would be happy to reuse all seals.

 

Today I took the faulty caliper apart and everything seemed OK apart from the fact that I could not work out for the life of me how brake fluid was supposed to travel from piston 1 to piston 2 via the split.

 

So I took the working caliper apart and 'hey presto' all was revealed.

 

In the working caliper, on the base of the piston housing, there is a small drilling that comes from the base of the bolt hole that is capped off with a bolt, this bolt has a hole in it that then allows brake fluid to travel to the bleed nipple.

 

Drilling is shown in the pic below on the left hand side of the piston base:

 

 

 

However, on the fault caliper, there is no drilling in the base of the piston housing!!

Therefore I deduce that this caliper was faulty from the factory and whoever had then before never had a working set of brakes!!

 

 

 

The guy I bought them off had said they came on the project car he had just bought and was selling them as he wanted bigger brakes / wheels (these are 7.5").

 

So if I drill the hole they will work?

[Ethel]

Also to add that you might need the piston on the side that the hose connects to slightly out before air/fluid would flow to the other one, when fully retracted its possible that it might block the hole to the other side due to tollerances in the calliper so check that as well.

A grease gun may work with with brake fluid, they've been used as improvised hydrolastic pumps.

[Tornado99]

 

The way the calipers are made is after casting the pistons are machined out. For the fluid tubes are drilled from

1. piston 1 to the hose connection,

2. from split joint to piston 1 

3.from split joint to bleed nipple and

4. from bleed nipple to piston 2. 

You have confirmed that both pistons are free and that tubes 1 and 4 are clear. If you split the caliper you will have full access to clean out tubes 2 and 3 with a small drill bit and confirm that you have the two correct halves. If you have no leaks so far and the units are fairly new I would be happy to reuse all seals.

 

Today I took the faulty caliper apart and everything seemed OK apart from the fact that I could not work out for the life of me how brake fluid was supposed to travel from piston 1 to piston 2 via the split.

 

So I took the working caliper apart and 'hey presto' all was revealed.

 

In the working caliper, on the base of the piston housing, there is a small drilling that comes from the base of the bolt hole that is capped off with a bolt, this bolt has a hole in it that then allows brake fluid to travel to the bleed nipple.

 

Drilling is shown in the pic below on the left hand side of the piston base:

 

 

 

However, on the fault caliper, there is no drilling in the base of the piston housing!!

Therefore I deduce that this caliper was faulty from the factory and whoever had then before never had a working set of brakes!!

 

 

 

The guy I bought them off had said they came on the project car he had just bought and was selling them as he wanted bigger brakes / wheels (these are 7.5").

 

So if I drill the hole they will work?

 

Great find. But unless you have confidence in how these are to be drilled/plugged etc to permit correct fluid flow, then I'd chuck them and get something made properly. Not a big expense for piece of mind.