master cylinder problems?!

[Matty]

Ive resealed my master cylinder (early type with lock nut on top) and when I try to bleed the brakes nothing happens?

I took the top brake line off and used a syringe to prime the piston, and when the pedal is pressed I get bubbles on the downstroke, then the fluid gets drawn into the cylinder on the upstroke.

I guessed that I must of put the seals in the wrong way, so ive stripped it down to have a look...and they were in the right way? I put them back in a tried the same test again on the floor, and it did the same thing...Am I missing something or do the very early cylinders have the seals fitted the opposite way round or something? It was a few months ago since I stripped it down and can't remember for the life of me how it was when I took it apart?

Cheers
Matt

[Austin Cooper]

I've found a section of the early type of master cylinder in the workshop manual (AKD1178D) I can scan it if you want, I just need to install the scanner. I don't know if there are any internal differences with the early type of cylinder or what is causing the problem though I'm afraid.

[1071bob]

This might help.

[Austin Cooper]

Hope this is of some help.

[andy1071]

Hi Mat,
When you normally bleed the brakes, you close the bleed screw when the pedal is down, this is because it takes some force ('vacuum') to cause the seal to 'collapse' and allow the fluid to flow over it as the pedal lifts/piston moves back, in the bore of the master cylinder.
If there is a lot of air in the system, then it's more difficult to get the seal to 'collapse'.
-Without the pipe attached you will not get this at all.

You could try pushing the piston down, and sealing the end with your finger when you let the piston return, to see if it is functioning.

Lockheed master cylinders work by having a very small hole ('bypass or compensation port' in picture) that is just in front of the seal when the piston is at rest.
The position of the piston is controlled by the ball end on the push-rod -the picture is not quite right.

So, 2 things to check:

1. make sure this hole is clear
this is not easy, as you can't get to it from the reservoir side, you have to poke a small (SMALL ) bit of wire in it from in the cylinder -not easy!
you can check if it's clear with:

2. make sure the seal is passing the hole, so the hole is on the 'system' side, not the 'back' side.
You can do this by blowing (low pressure air line) in the pipe end of the master cylinder, and listening to hear hissing in the reservoir.
This will also check that he hole is clear.
As I said, it's only a small hole, so there will not be much air flowing through it.

There is also a small valve at the outlet from the cylinder. This holds a pressure in the system ( 2-4 psi) to help stop the pads from moving away from the disc.

Hope this is a help?

/Andrew

[Matty]

Thanks for the replys guys, thats helped out ALOT!

Ive had chance to have another inspection of the master cylinder, and it turns out there is loads of grooves along the bore.

Ive refurbed another cylinder, and that works a treat...took a while to get the fluid to start mind.

Ive been looking into modifying a later cylinder to accomodate the nut at the top, and it looks doable, if its ok to have 2mm less on the stroke? Has anyone tried this before?

[mascher]

I don't see any reason you would have 2mm less stroke. As the worst, you would have to cut down the spring so it doesn't bind too soon and even that probably wouldn't be necessary. You could counter bore the cap to allow the spring more space.

Kelley

[andy1071]

Losing 2mm off the stroke shouldn't be a problem. -The working range is usually only the first 2-3mm anyway. -If the pedal is going to the floor in normal use, then I think you'll have other worries....

-And the 2mm lost, will only make bleeding the system take fractionally longer.

-Kelley,
I don't think you will be able to counter-bore the cap, as the non-return valve is there (and you will just start to lose the threads of the cap).
I wouldn't even bother to try and shorten the spring. -It's good to keep the spring force there to return the piston and seal past the by-pass hole.

[mascher]

Without making measurements I can't say whether counter boring the cap or cutting the spring would be necessary. The point I was try to make, poorly, was that since there is a spring between the piston and the cap there is little chance that there would be any loss in the stroke at all since the spring would simply compress an additional 2mm. A tapered spring is unlikely to bind.

Kelley

[nick rogers]

Modifying later cylinders is an interesting suggestion and in fact I have a couple of the screw-in inserts NOS. I can think of a couple of possible problems. One is that most later cylinders taper in at the top and the other is that early cyls were 0.75 and later ones .7" bore. Good luck with it however.

[bill773mini]

Modified NOS master cylinder fitted to one of my 59's to incorporate the screw top.




When fitting them I always prime them with a finger over the exit hole to build up pressure, without doing this you get nothing. I've done the same assuming that there is something wrong with the valve so removed and stripped them down again to find nothing wrong. They just need priming first.

[Matty]

Ah so it looks like it can be done! All I was going to do was cut the tapered section off, the ream the cylinder to .75 just to the thread depth, and tap 3/4unf (Looks like that size).

Ive bled the system now using a vacuum pump (which worked very well on my friends mini), but I still can't get pressure? If I pump the pedal there is no resistance, but if I look into the master cylinder I can see bubbles rising up through the fluid?

Ive tried removing the brake line that comes out of the top, then pressurising the master cylinder by blowing down a pipe into the where the cap goes and fluid starts to come out of the top. After doing this the pedal seems to pump the fluid out of the top, but once the brake line is refitted I get no pressure again? Im completely stumped on this one?

[andy1071]

Hi Matty,
Silly question: With the mixing of cylinders you have been doing, you do have a .75" seal in a .75" bore, not a .7" in there?

Apologies if i offend you with such a simple question, but it can be done....
(The Lockheed seals have their size moulded in, so you can easily read it)

Also remember that the by-pass port and the feed port are part way up the cylinder, so you need to keep the reservoir nearly full all the time, otherwise you will suck in air. It's a bit of a pain to keep filling, that's one of the reasons why I tend to use an Eezi-bleed.

-I know lots of people hate them, because the seal of the cap onto the reservoir can be a bit dodgy (and you pump loads of brake fluid over your nice paintwork ), but I find it very useful. -And it means you can bleed the brakes by yourself.

/Andrew

[Tim]

Ah so it looks like it can be done! All I was going to do was cut the tapered section off, the ream the cylinder to .75 just to the thread depth, and tap 3/4unf (Looks like that size).

Didn't the later tin-tank clutch master cylinders already have a .75" bore? You'd neet to make sure they had the brake non-return valve fitted. I don't know if the repro ones are avialbel with a .75" bore, and its possible that their reservoirs have a different orientation. I wonder if its practical to use the reservoir off a repro m/c or even a good second hand later one to repair an early m/c?

Tim

[Matty]

Andy - Its a fair enough question, but I definitely have the right seal in, as im now using a .70 master cylinder as the .75 was scored. All the seals and bore looked in good condition when I stripped it down.

Is there a way to prime the master cylinder after its been fitted?

[andy1071]

I think using your method of blowing down a pipe into the reservoir is as good as any.

-I'm guessing you've had a fair bit of practice already

Everyone has their own method for bleeding the brakes. Some say: slow down, fast up; build up pressure then release bleed screw; fast down, fast up. And every other possible combination.
-I was always taught (I'm ex Lockheed / AP Racing), slow down and medium up (and open/close the bleed screw). -I don't like to think of the fluid being pushed at speed through all the joints/pipes etc, as they were never designed to 'flow', so you can get a lot of turbulance and cavitation.
(As I now use an Eezi-bleed, I can control the flow rate by the bleed screw).

If I'm working on a master cylinder, before removing it I usually remove the pads from 1 caliper and pump the pistons out (add a block/screwdriver so they don't pop out), so when I re-fit the m/cyl, I can push the caliper pistons back and 'reverse bleed' the master cylinder. Then I don't have to push the air all the way to the bleed screw.

-Sorry, I know it's a bit late for you now

/Andrew

[Matty]

I think using your method of blowing down a pipe into the reservoir is as good as any.

-I'm guessing you've had a fair bit of practice already

/Andrew

Haha!

Once the fluid as reached the master cylinder outlet at the top, should it be able to drain back into the reservior after the pedal is released? As this is what seems to be happening.

Ive got a feeling the master cylinder is going to have to come out again. I might try purging the master cylinder using a piece of old brake line attached to the top of the cylinder, and see if I can isolate the master cylinder and see if I can get pressure with nothing else attached.

Its always the 'simple' jobs that seem to take the time!

[andy1071]

Hi Matty,
Sorry for not replying sooner, I don't have t'internet at home at the moment, and had Friday off.

If the non-return valve (it's actually the bit of spring-steel in the plastic spring-locator in the end of the cylinder) is removed, the fluid will flow back into the reservoir to level up. It shouldn't flow back in with the valve in place. -Though as there is no pressure in the system, then the 'seal' will probably not function as intended, and allow the flow.

-The system should operate as normal without this valve, but there will be the risk that the pistons in the calipers may suffer 'knock-back'. -This is where the play in the wheel bearings, disc flex etc, push the pistons back in the calipers. -Not fun when you go to brake next time....
-In race cars, springs are fitted behind the pistons in the calipers to stop them suffering knock-back. -Different 'weight' springs are fitted for different applications, so you don't create too much drag from the brakes.

You could fit a bleed screw dircetly into the master cylinder, bleed it, and then see if it holds pressure?

[Matty]

Not to worry Andy...I think ive sussed it.

When I swapped the master cylinder over I lost the clevis pin down the crossmember section, so I used another clevis pin (slightly smaller) as a temporary measure. It turns out that as I have been bleeding the brakes and testing them the pin has come out... The pedal has still been working, but not quite pushing the cylinder far enough. Since fitting a new pin (correct size) I could feel the pedal pump up the first few pushes, then went relatively solid. Ive spun each wheel then pressed the pedal, and it stop them solidly. They probably just need another quick bleed then they should be good to go!

Thanks for all the replies, they have helped out alot. I'll have a look into modifying my other spare cylinder now they are all working.

[Tim]

I had the same problem with a clutch M/C once. I couldn't get it to clear - no matter what I did with the hydraulics. I ended up putting washers between the pressure plate and flywheel to make it clear properly, then I discovered the clevis pin wasn't fitted. Once I put the clevis pin back in it was the lightest clutch in the world!

I started the stripdown and rebuild of my brake M/C last night. There is a lot of useful info in this thread.

Tim