Master Cylinder

[Woodyalan]

I've just got my traveller on the road after restoration. Unfortunately I'm finding the brakes are binding on all four wheels so have removed the master cylinder (new). On removing the cap there are two ports visible the rear one being smaller. I can just poke a piece of welding wire through it into the bore but nothing thicker. I'm wondering if the hole is enough diameter to do it's job or does this suggest it is blocked ?
Thanks
Alan

[simmitc]

If you have overhauled and replaced everything, did you adjst the yoke on the pedal rod to ensure that there is freeplay - if the piston cannot return fully then it would keep pressure in the system.

[Woodyalan]

The pedal did seem overly firm so I adjusted the rod to increase free play but it has made no difference to the binding.

[bmcecosse]

Try slackening back the adjusters very slightly to give the cylinders a little movement and see if that helps.

[Woodyalan]

I have tried that to no avail. If I take the car for a run when I get back all the wheels will be stiff to turn by hand, after they have cooled down they will be back to normal. Does anyone know what diameter that port should be as it seems too small to me?

[bmcecosse]

Is the return spring on the pedal (inside the chassis leg) connected ok?

[Woodyalan]

Yes the spring is fitted and working. Sorry to keep harping on about the ports inside the cylinder, my theories are usually wrong anyway, but the front hole will take a 4.5 drill bit whereas the rear hole will take nothing thicker than welding wire. If someone has an old cylinder lying around maybe they could check to see if this is correct or should the rear hole be bigger. Another possibility is that I could be overfilling the cylinder. I fill it to the bottom of the neck whereas the workshop manual shows the level at halfway up the reservois chamber. Could this have a bearing on the way it works?

[bmcecosse]

A hole is a hole - it's there for a reason- pretty sure any 'backpressure' will leak away in a second or two with a 'welding wire' hole. Yes - you have overfilled a bit - but I doubt that's the problem either. Is the breather hole in the cap open? Why did you change the master cylinder anyway? May give us a clue.... And were all the slave cylinders renewed too?

[Woodyalan]

My ever helpful bit of welding wire will go through the breather hole quite happily so that's ok. I'm wondering if there had been any blockage in that small port whether I might have shifted it with the wire, won't know till I put it all back. The old cylinder, which I've since thrown away, was not holding pressure, the pedal would sink to the floor. I've renewed the cylinders front and back on the near side for the MOT. The off side cylinders are unchanged but the binding is affecting all wheels equally suggesting pressure must be remaining in the system after braking. I could try extending the free play at the pedal further but it's already got the 3/4 inch it's supposed to have.

[bmcecosse]

As long as that is genuine free play - before it starts to move the master cylinder internals. There have been reports of 'dodgy' new master cylinders , but I believe that related to not holding pressure - rather than holding too much pressure! The system is designed to hold ~ 5psi pressure to hold the shoes 'near' the drums - to reduce pedal movement. I'm guessing all the return springs are correctly in place. So - after a run - is the pedal then 'hard' and scarcely moving when you press it. But on return later, it sinks down much further before applying the brakes?

[Woodyalan]

I didn't note what the pedal was like after a run, only that the car became stiff to maneuver and on jacking up all the brakes were binding. At the moment I'm just trying to cover all possible faults before putting it back together. If the problem remains after assembly I suppose I could try gradually increasing the free play to see if it reaches a point where the problem goes away. Clearly I can't see what's happening inside the cylinder when the pedal is operated so can't know when the piston actually starts moving.

[bmcecosse]

No - but you should feel it.

[Trickydicky]

Have you replaced all the flexible hoses? If not after a long layup they can cause problems like this as they perish internally.

[Woodyalan]

Yes all hoses and all pipes are new

[amgrave]

Wind all the adjusters off one click. If the shoes are rubbing they create heat and this expands them a little and the expansion will press them onto the drums slightly more than when cold, hence after a run they bind a bit.

[Woodyalan]

I emailed Rob Thomasson our tech guru and he confirmed that though the MC is new the symptons are of a blocked transfer hole, the smaller of the two holes in the reservois. The hole is around 1 mm and is stepped (the top part is larger). I probably cleared any blockage when I poked my bit of wire through. Does seem like asking for trouble to have such a tiny hole doing the job it does, anyhow the MC is back in now and I've taken it for a run and happily there is no binding.

[bmcecosse]

Great success then !

[Woodyalan]

Yes! Now what's the next problem?