mk1-forum.net

hi sirs,
i am converting the drum to 7.5" disc brake at front wheel with 0.7" brake master cylinder (longer can version with non-return valve inside). after fitting the disc and having bleeding the brake, i found the problem that both calipers are locked up. (calipers are recondition with new seals and stainless piton about 2 years ago). the calipers unlocked when i released the bleed nipple. i am not sure that i need to take this non-return valve off or not when installing the front disc brake (because the non-return valve will lock tge caliper or the cailpers are seized due to sitting period before installation

thank you so much and warm regards
ink

Look at the calipers: are the bleed neeples on the top or bottom?

check that the caliper rubber hoses are in good shape and not restricting the return flow of the brake fluid.
the hose can perish internally but not leak on the outside.

The non-return valve in the master cylinder is designed to only give a low system pressure (around 4psi).

Its purpose is to stop the pistons (in drum and disc brakes) from 'drifting', vibrating, or otherwise backing-off.
It is normally possible to push pistons back (such as when changing pads) with the valve in place. So if the brakes are 'holding-on', I would be inclined to look at other causes, such as those suggested.
-But of course, it is possible that the valve is faulty -though I have never heard of one giving problems.

hi sirs,
thank you so much for all replies.

i use goodridge stainless steel hoses and it is new. when the calipers locked, i also tried to release the front brake pipe at 3-way connector. after releasing the pipe, the caliper unlocked promptly. i think the hose should be ok. (any advises)

by the way, is it possible to be air lock inside? or caliper seized? or rear brake not adjusted properly? do you have any processes when installing new parts all frint and rear?

warm regards
ink

matic wrote:by the way, is it possible to be air lock inside?

Yes, even more if you fitted them the wrong way.

ricardo wrote:Look at the calipers: are the bleed neeples on the top or bottom?

Check this first, it's a common mistake.

hi ricado,
both caliper bleed neeples are in the top position.
regards
ink

Check if the piston on the master cylender goes back/down enough ? If it dont, it holds the pressure.

There is a small hole in the side of the bore, and the lip on the rubber seal is suppose to sit UNDER this hole, when pedal is back in neutral position (no foot on it)

Its very easy to find out = just loosen the brakepipe on top of the mastercylender, and see if brakefluid poors out ?

Jens Christian

hi jc
yes, i partly remembered that when disconnected the top pipe at the brake master cylinder, the brake fluid poored out from there. this means the piston does not sit properly, isn't it?
regards
ink

matic wrote:hi jc
yes, i partly remembered that when disconnected the top pipe at the brake master cylinder, the brake fluid poored out from there. this means the piston does not sit properly, isn't it?
regards
ink

It means it dont go back/down far enough, to clear the little hole in the bore.

First you pump up the brakepressure - then loosen the 2 nuts that hold the mastercylender to the bulkhead, and see if this helps ?

If it does = you need a gasket/plate under the mastercylender, but if the pressure is still there (with loose mastercylender) you need to grind a bit off the top of the pushrod,
so the piston goes back enough, for the lip, on the rubber seal to clear the hole in the bore.


Jens Christian

As JC has said, the most common cause I've found is because the MC is not fully returning for one reason or another, sometimes it's that coconut hair that they seemed to use as sound deadener, sometimes an adjustment shim is needed under the MC. Also, be sure you have the correct pedal to go with your MC.

Another thing I've found with some brand new calipers is that the rubber grease seems to have dried out (if any was used) on the seals and so the pistons stick. I find removing one pad at a time and pumping the piston 1/2 to 3/4 the way out and then pressing them back takes care of that.

The other thing to check and it might sound silly, is that air hole on the lid of the master cylinder is not blocked. If it is blocked the breaks will stick on and will overheat the fluid and pads.

The position of the piston & seal in the master cylinder is controlled by the spherical end of the pushrod, the washer on the pushrod, and the circlip.
If the seal is not returning past the cut-off port (the small hole), then I would find out why.

-I have never found a problem with a proper Lockheed cylinder (and being an ex-Lockheed person, I don't think I'm biased ). But after-maket seal kits, and 'replica' cylinder are not always to the original standard (or dimensions!).

I would not recommend modifying the end of the pushrod (I'm sure it all works fine for you Jens, but just that I wouldn't do it).
-I usually find that if the piston is not returning in the cylinder fully, it is being held down by some external cause, such as Spider and Jens say.

-It could be something as simple as the pedal return spring has broken, or 'gone weak' (try pulling the pedal up).

You say the calipers had new seals and pistons 2 years ago, have they been used since then, or 'sat on the shelf' -As Spider says, they might need actuating fully to lubricate the seals a bit.
-The seal friction on the piston of the caliper is also important, so if the stainless pistons are not ground then the seals could be 'grabbing' the pistons. The stainless pistons I had in my calipers were perfect, but again, quality can vary....

andy1071 wrote: -It could be something as simple as the pedal return spring has broken, or 'gone weak' (try pulling the pedal up)

Thanks andy1071, that's reminded me of another cause I did once find!

The owner had diligently removed and overhauled his pedal box, certainly did nice job, however, one mistake he did make was to put the pedal spring in back to front. Also made the clutch near impossible to bleed!

andy1071 wrote:
I would not recommend modifying the end of the pushrod (I'm sure it all works fine for you Jens, but just that I wouldn't do it).

I had no reservations what so ever, as it was my clutch mc that acted up, and it was only 0.2mm I grinded off.
I fully agree = always take great care with brakes, especially a one circiut system.

hi sirs,
thank you so much for all comments. i will try to give more info. to you all. (i forgot to take the photo to show you)

by the way, i will list dow the problem.
-calipers both had been overhauled and been sat for 2 years. the piston is stainless steel which i let the local machine shop to copy from steel to stainless. i am not so sure how quality it is.
-i have an eye check. when i took the brake pad out, i have seen the pistons came out but not too much around 2mm
-i have found another problem. when i pump the brake pedal, the brake fluid leak from the pipe thread at the brake master cylinder. i tried to tighten it more and it still leak from there.
@ i have not tried to unbolt the two nut, hold the madter cylinder.


ps. i will try to take photos and show you all.

regards
ink

Long shot. Any chance it's not a Mini master? Didn't the ADO16 masters look the same but didn't they have a longer actuation rod? A longer rod would bottom out in the firewall before travelling as far down as it should.

Hi Sirs,
I attached the photos via the below link in which you all can download and see the problem that i have.

https://www.wetransfer.com/downloads/9c ... 159/e5c875

I think i have a mismatched master cylinder in which the push rod is a longer version. When I unbolted nuts and took the pin off, it promptly released the pressure and unlocked the caliper. However, I still have the problem of brake pipe leaking. You can see in the photo, which it came out every thread bolt.

Do you have any recommendation to fix it?

Regards
ink

Hi, your link wants me to sign into or download something, so I haven't had a look but if all or some of your threaded joints are leaking, I would suspect poorly formed brake pipe ends, did you flare them? Or did whoever made them understand where single and double flares are required?

You may have purchased completed pipes, in which case, your problem may be elsewhere. Maybe seek help from a brake shop, it pays to get them right!!

hi sir
some pipes are new from uk supplier, some pipes are made new by me and some are original. it is all correct flare head and unf thread. i am still not sure why it leaks badly. is its pipe not seated properly?

ps. i try to attach the photo below

regards
ink