Master cylinder disassembly

[Nurse Julie]

All the original Honda SOHC/4 master cylinders are completely metal, yours is plastic, so probably from a later non CB SOHC/4.

[ Guests cannot view attachments ]

[ Guests cannot view attachments ]

[ Guests cannot view attachments ]

[keithtraffic]

Thanks Julie, I see what you mean, but is that likely to have any real bearing on the issue I have?

[Nurse Julie]

Only in as much as you have a kit for an original SOHC/4 master cylinder and your master cylinder is not that original part. To me, this would be a problem as I would only fit the correct overhaul parts to the correct cylinder. I wouldn't know if yours is compatible or not but, personally, I wouldn't risk it.

[Bryanj]

First time i have had chance to look at pics and the new seal is what the original would have looked like when new, as to fitting the seal needs to be well lubed with brake fluid WEAR GLOVES. Worries me difference in length.

[keithtraffic]

Umm -doesn't sound to good to go ahead with the master cylinder I have - does anyone have a puka 550 one that they would like to sell? I obviously have a suitable re-furb kit for one so could apply that if it was required to bring the cylinder up to scratch.

[Trigger]

One on Ebay >>> https://www.ebay.co.uk/itm/Honda-CB750-FRONT-BRAKE-MASTER-CYLINDER-LEVER-CB500-CB550-F-Series-K-Genuine/233321059302?hash=item365304d3e6:g:hVwAAOSwu~FdYDg-

Or just buy a replica one from DSS

[JezzaPeach]

Also look very carefully at the bore. When I did mine I reverted to a copy one from DSS.
My original had very slight corrosion and even after polishing I wasn’t confident it would seal perfectly.
This followed me putting a new seal in the calliper which had very slight corrosion in the seal groove and leaked. So again got a good copy new for £40 after some looking.
Basically the lot is now mostly new, shiny, and works well (with EBC FAO13V pads £33.95)
Cost adds up but it’s the brake!

[keithtraffic]

Thanks - have been in contact with DSS who have been very helpful and have tried to match the dimensions of the piston with what they have in stock to see if they could identify exactly what master cylinder I have, but unfortunately they could not really pin it down.

I have concluded that I will now buy a replica master cylinder kit from DSS as the cost is not too high and also (to their credit) DSS have said that if I send the replacement repair kit back to them they will refund the cost - which I think is very good of them!

[paulbaker1954]

Wise decision, this is your life you are messing with here 👍👍👍

[keithtraffic]

So the brake saga continues!

I have received my new replica Brake cylinder from DSS - looks very nice and have fitted it.

Once fitted I was able to pressurise the system and force out the piston from the wheel cylinder (I has suspected that some of my brake issues were down to the pad itself not moving back when the brake was released and therefore binding on the disc). I found that the pad itself was almost held in place by awful looking 'gunk' and would not slide easily on its own.

I am replacing the pads, but does anyone know if the build-up of any sort of 'gunk' around the pad is normal and should there be something to stop it? (I have seen on the packaging of the new pads that it is recommended that they are removed and cleaned every 3000 miles, which doesn't seem a lot, but there isn't any mention of this in the Honda workshop manual I have, nor the Haynes manual).

I have also read about using copper or silicone crease behind the pad but have assumed that nothing should be smeared around the edges of the pad to ensure that there is no possibility of any grease contaminating the pad surface or the disc itself. Any thoughts on this?

Also, does anyone know the mechanism that causes the pad to release once the brake handle is released - is it actually pulled back by reverse hydraulic pressure or is it just naturally pushed away from the disc by its rotation (I know from the manual that the spacing between the pads and the disc is meant to be pretty small (just 0.15mm)!

[Bryanj]

Remove the paint from the outside edge of the pads and the "gunk" inside the hole it fits in, perfectly permissible to use copper greas on the edge and rear.
The only thing that pulls the piston back is the "twist" in the rubber seal, the groove is machined such that the seal becomes diamond shaped and only one corner is on the piston, this means that as the piston moves the seal twists until a max point then the piston slides through, it is this deformation that pulls the piston back on pressure release and only the "looseness" of the pad in caliper that gives clearance

[K2-K6]

[ Guests cannot view attachments ]

A little schematic to illustrate the above,  bit fuzzy but shows what happens.

Surprisingly little has changed in caliper concept even since these early types as most current calipers have exactly the same arrangement regarding principle seals.

What generally changes is that most employ a weather resistant seal outboard of the main piston seal operation to keep them clean,  hence the need to carry out frequent maintenance on these old one's, especially if used on salted roads.

Silicon grease around that seal to piston interface is as near as possible to waterproof which helps to keep them from corroding.

[Bryanj]

Forgot to mention that the seal groove needs to be scrupulously clean for the seal to work properly. N.B. a dremel brush will only "polish" the crud in the groove you need to dig it out with a dental pick or some such.
That little diagram is ideal except that the seal becomes diamond shaped as the groove is not flat bottomed, even more teason to make sure the corners are cleaned out well.

[K2-K6]

As a measure of effectiveness from working on brake calipers,  the rear calipers on many VW cars are almost exactly the same proportion and material in respect of aluminium caliper and hard chrome piston. But with a dust seal outer to the principal hydraulic one,  they still eventually seize after about 100,000 miles which you can first feel through the handbrake being ineffective or failing to release. Plus lowered efficiency when mot tested.

Even with an assisted foot brake they are difficult to push the piston out which shows how much drag any corrosion effect has.  I clean these and reassemble using silicon grease for both placing the hydraulic seal into the caliper body,  plus reinserting the piston.  Most calipers you can just push the piston in with your thumbs when cleaned and greased,  they shouldn't need much force to do this and if they do it indicates that some more cleaning of the seal groove is required.

Testing after doing this always shows absolutely equal % on mot rollers plus efficiency at maximum end of tolerance for handbrake,  showing effectiveness.

Edit ;- my reasoning for using silicon grease in those locations (Honda states to use it in their manual too)  is that it excludes both brake fluid and moisture from that area at which corrosion compromised the piston operation.  It gives longer service life like that than I've found with other methods.

[keithtraffic]

Many thanks for the comments.

I have seen the note about silicone grease in the Honda manual so have purchased some of that and (perhaps luckily) I had already thought to replace the piston seal so have a brand new one.

It is still quite remarkable that its only this that brings the piston back to move the brake pad away from the disc!