CB750K0/Sandcast Master Cylinder Differences from Later SOHC/4 models.

[AshimotoK0]

Does anyone know what the differences are between the body of the CB750K0 brake master cylinder and the late 500/4 400/4 variants?
I know that the  early 750 had the adjustable stop arrangement and the later one had the rubber cushion insert and plate arrangement. Plus the cap on the early bike was dished on top and didn't have the DOT emblem on it. However, by tapping the hole in the later one I dare say you could replicate the original stud and lock-nut arrangement on a later body.

I have an early 750 one with decent un-pitted bore but some idiot PO has drilled out the tiny hole inside the reservoir. I wonder if that could have an insert put into it with the same size hole dia. There are brass inserts similar to this in the tops of Honda CV carbs (under the diaphragm/piston assembly). I  think I read somewhere that Vic World has master cylinders re-lined but never heard of a repair on the tiny hole in the reservoir chamber.

[mickwinf]

interested to know how drilling the small hole would affect braking, the hole is so small i struggled to find something small enough to clean mine out.

[K2-K6]

A larger hole gives increase in lever travel.

The first seal, main pressure one, has to pass fully the reservoir hole to then build hydraulic effort in the outgoing line, any further travel to achieve this gives longer lever stroke before the caliper is moved. Also dependent on how far the primary seal clears that hole at rest as that distance is redundant before the hole is covered.

After the primary seal has  passed, the hole then is open to the piston bore (non pressure) with the secondary seal clearing that void as the piston moves further.  Without that second seal it's hard to stop fluid leaking out the back end of mastercylinder.  It's this second action you see push fluid into the reservoir if you squeeze it with cap off as it clears low pressure fluid from that bore.

Bigger hole would likely wear the primary seal more as it doesn't rotate but always pass linearly over that port.

[Piki]

interested to know how drilling the small hole would affect braking, the hole is so small i struggled to find something small enough to clean mine out.

This weekend I was trying to repair mine, it was completely clogged, I used a very fine copper wire from a motor winding