Brake master cylinder reaction valve spring

[paulsco]

I had my master cylinder reconditioned some time ago by a reputable company, who re-sleeved and rebuilt it.
I have not driven the car since as I was still restoring it and had not filled the system with brake fluid.
I noticed that the reaction valve was VERY close to the engine frame and while investigating I removed the large plastic reaction valve cover.
I found a large spring behind the diaphragm and no small spring in the attachment at the front of the plastic housing.

This is the spring I found:







I found another thread on Jag Lovers where someone else had found this larger spring, but no conclusion was reached.

I have now removed this large spring and put a smaller spring in the nose of the reaction valve as per original.
I note that the spring is not included as a spare part anywhere, so I have used a generic spring.

My question is: is the small spring rate critical and has anyone ever seen a lager spring used behind the diaphragm?

Thanks,

Paul

[42south]

Hi
There was a solution offered to reaction valve problems that involved fitting a slightly stronger spring than the original.
I think it was to ensure the small piston that the reaction valve operates will always return. If it doesn’t you can get sticking brakes.
Your spring may be an attempt to implement this idea.
Cheers
Mark

[paulsco]

Hi Mark,

Thanks for your suggestion.

The only problem with that theory is that the large spring is pushing in the opposite direction, away from the master cylinder, i.e. helping the small piston to push outwards, where as the small spring is pushing it back the other way.

Paul

[mgcjag]

Hi Paul.....why not contact the company that refurbed it and ask why its there....not sur what you mean small spring.....there shouldent be any spring in the reaction valve from the diagrams..Steve

[42south]

Hi Paul
Sorry, I misunderstood where your large spring was. That is most unusual.
I was talking about the small spring in the nose of the valve, it’s not shown in the diagrams, but can be accessed by popping the nose off the end of the valve. This is the one that some recommend to replace with a slightly stronger one for better reaction valve operation.
I’ll be interested to hear the reason for the large spring.
Cheers
Mark

[mgcjag]

Hi Mark.....re the reaction valve piston operation.....there is a shaped flap that pushes on the piston( inside the cylinder)....if you look closely you will see it looks as if its been stamped out of a sheet.....so has rounded edges one side and sharp edges the other......i found that taking of the sharp edges on the one side made for better operation on my car it was the flap sticking and not the actual small piston.....Steve

[paulsco]

Hi Steve,

The small spring I am talking about is in the nose of the reaction valve cover.

https://redirect.viglink.com/?format=go ... f%3Fdl%3D1

Page LY.s.8, part 38

Yes it might be a good idea to contact the company. I just wanted to know if anyone had come across this before. The large spring doesn't seem to have a purpose to me, so replacing it with the small spring seamed a good idea.

Paul

[paulsco]

Hi Mark,

Thank you for your additional information.

Paul

[mgcjag]

Hi Paul.....sorry to have added confusion...i understand what your refering to now......Steve

[xk.app.soc.]

Hi Paul.
I had my M/S cylinders and servo unit refurbished and re sleeved by Past Parts. I also found they had fitted the large spring. The reason I found it was because while I was bleeding the brakes, the brake pedal pumped fine, but when the engine was running and I had vacuum the pedal was solid.
When I removed the reaction valve cover I found the large spring hadn't been fitted properly. (see photo)

When I refitted the spring I could then pump the pedal with or without the engine running.
As I couldn't locate this spring in an exploded diagram of the MC. I called Past parts and asked them why it was there, he said when he re-builds this type of MC he always put these springs in to help prevent brake drag.
The brakes work fine with or without the large spring.
Just wondered what you did, did you keep the large spring, replace it with a smaller one, leave it out

Cheers Phil.

[paulsco]

hi Phil,

It was Past Parts that did mine.

I left the smaller replacement spring in and it seams to work fine.

I still fail to see what the larger spring could do; my understanding is it needs to push on the centre piston in your photo and the larger spring clearly can't do that.

Paul