24 replies to this topic

[John K Series]

Thank you for info. See pics
https://ibb.co/bMcLWNX0
https://ibb.co/xSgLzCH7
Im sure the shims are only there to hold the rubber bungs off the holes like you say. The hollow pin just goes into the cylinder like the other hole next to it. I think i'm fine with where they are now and like Spider said it doesnt matter which way round they are. Happy days. Sorted

[Ethel]

...just had a thought (not even wearing my Columbo mac). If that actually is a travel limiter, could it be to balance up the different bores in the event of a circuit failing? The, bigger bore, lower piston won't have to travel as far to displace as much fluid as one the same size as the top piston. So, it could make metal to metal contact with more reserve travel given over to the, hopefully still working , upper piston. The assumption being the pistons are standard parts between all master cylinders of a particular bore.

[Spider]

...just had a thought (not even wearing my Columbo mac). If that actually is a travel limiter, could it be to balance up the different bores in the event of a circuit failing? The, bigger bore, lower piston won't have to travel as far to displace as much fluid as on the same size as the top piston. So, it could make metal to metal contact with more reserve travel given over to the, hopefully still working , upper piston. The assumption being the pistons are standard parts between all master cylinders of a particular bore.

 

My concern back on post #11 is that this screw may well limit the travel of the Primary vs the Secondary Piston.

 

For the benefit of John K Series,,,,, The way these (and just about all Tandem Masters on most other cars) is;-

 

 

 

1. When The Pushrod is pushed, that in turn moves the Primary Piston, this will then displace fluid to the Rear Brakes (via the lower port) and also to the Secondary Piston.

2. As pressure starts to build in the Rear Brakes, this then moves the Secondary Piston via a hydraulic coupling between the upper seal on the Primary Piston and the lower seal on the Secondary, however at this point, there's not enough pressure for operation of the Brakes.

3. This will then build pressure in the Front Brakes, however it is hydraulically balanced between the Front and Rear Circuits via the Hydraulic Coupling between the Primary and Secondary Pistons. This then brings on the Brakes in the usual way.

 

If there is a failure in the Rear Brakes;-

 

4. Since no pressure will build in the hydraulic coupling, the Primary Piston will move up until the extension on the upper Piston makes mechanical contact with the Secondary Piston

 

 

5. Via this mechanical contact, the Primary Piston directly moves the Secondary Piston to (theoretically) bring on the Front Brakes, albeit with a lower than normal pedal.

 

If the Front Brakes Fail;-

 

6. The Master operates in the normal way, except that since no pressure will build in the Secondary part of the Cylinder, the Secondary piston is pushed all the way up until it's Piston Extension contacts the upper part of the Cylinder, then braking pressure will build in the Primary circuit. Apart from having the assembly work correctly, this too is why it's important to fit the lower seal on the secondary piston inverted to all the others, because if it's fitted the same way, in this failure mode, you'll have no brakes.

 

 

 

My concern here with adding the screw in to the extension of the Primary Piston is that may well make mechanical contact with the Secondary Piston before Primary Pressure builds to normal braking pressures, thus in a very crude ad unmeasured way, limit Primary pressures, it may well be to the point that the Rear Brakes are ineffective.

[Ethel]

.... kinda adds to my suspicion, the GMC227 having the primary & secondary swapping places.  Dubious a little bit of rolled sheet would be up to the job though.

[Spider]

.... kinda adds to my suspicion, the GMC227 having the primary & secondary swapping places.  Dubious a little bit of rolled sheet would be up to the job though.

 

Yes, there's that too ! It's supposed to be at the bottom and plays an important role, though it's something I haven't had to think about since my engineering days, so right now, I can't recall what that is !

[John K Series]

Hi Ethel and Spider. I thought I was OK with this cylinder on 13/03/2026 leaving the screw out. To confirm, the car does have a brake limiter valve fitted separate.
Your discussion after 13/3 went over my untechnical head, sorry!
So am I leaving the compressing bolt/screw out? Or having it to bits again to put it back in?
Cheers John

[Spider]

I am suggesting to leave the screw out.

[John K Series]

Ok thanks

[Ethel]

I'm not clear which bit you're referring to as the screw. There certainly shouldn't be anything that will prevent the springs from rebounding after they're compressed.. If you refer to Spider's exploded diagrams, there should be two retainer clips, one  between each spring & piston rod - number 10.

[John K Series]

Hi Ethel,
To confirm, "the screw" i referred to is in my first post of the thread. My primary piston has Spider's diagram part 10 (spring retaining clip) which does as it says on the tin- a clip that retains/holds the end of the spring in the cylinder bore. On mine though the retaining clip had a screw seating on end of that clip and into the primary piston- almost compressing the spring fully (see my pucs). Screw now removed as per advice.

Ok, its all back together now.

Anyone got a link to a video of bench bleeding this type of brake cylinder?

To confirm- part number GMC227- the vertical piston vertical reservoir yellow tag brake master cylinder?

I have google/youtube but all I get is horizontal dual cylinder bench bleeding videos. These are not usable as a guide for this type of cylinder.

Thanks in anticipation :)