Re-set brake fault valve & switch on 2 circuit 121
 

Hi all,

My brake light is on after bleeding the brakes without removing the switch. My mistake as I have learned from the research I have done. A question on how to move the shuttle valve back to center position: Can I move it by bleeding one of the caliper or wheel cylinder bleed nipples on the other circuit? I will do this with the switch removed of course. However I don't know how will I know it has centered, and not moved across to isolate the circuit being bled?

The position of the shuttle valve block with switch is awkward with the switch facing down. It is not possible to see the positon of th eshuttle valve wihtout a borescope. I am tempted to install it upside down with the switch on top so I can see the shuttle valve and see if it is centered. Has anybody done this? Can you see any problems with installing it this way?

Thanks,
Chalrie Perry 69 121 B20 with dual circuit brakes

Brake valve
 

With the switch removed, provided the 2 brake circuits are free from leaks and the pressure in the circuits are the same, the shuttle valve should self centralise.
My late amazon has this unit fitted and the switch does face upwards towards the bonnet.
It was fitted this way when I took over ownership in 2004.
If your shuttle does not self centre, you can slide it easily with a small screwdriver.
With the switch removed, operate the brakes and check the valve remains central.
Sometimes bleeding the brakes with the switch in place can break the switch operating plunger.
Good luck.

My 142 is fitted upwards as well. You'll need to mod the brake lines a little to change it round as they are higher when it's pointing up.

Charlie;

I don't think there are any design or construction, or pressure factors which require the switch block to be in any particular position or attitude, so the position you find it in, especially with a retrofitted one, is strictly how the plumbing worked out when the installer located it...having said that, he didn't make it any easier to inspect by mounting it "upsidedown"...as 133 notes, Shuttle "should" self-recentralize as it has springs on either side, but may/typically needs a bit of energy/persuasion to "unstick" and encourage this...try jabbing, sharp risetime inputs on the Brake Pedal (I don't believe I'm saying this, but some additional shock energy ("Lucasfix"!) inputs in the form of light tappy-tappys with a soft hammer while a helper is applying the Pedal energy inputs, may be the trick)...
https://www.sw-em.com/Two_Wr7markup.jpg
...BUT FIRST: Remove the Switch from the block! Having moved from the central position, and extended the plunger during bleeding is not when the plastic peg gets damaged/pieces sheared off, but when returning it and compressing it with not much of a ramp on the Shuttle IS...see also: https://www.sw-em.com/Two_Wrongs_Mak...g_Valve_Switch
https://www.sw-em.com/Brake_Warning_...es_new_old.jpg
...so how would you know if your Pedal jabbing inputs have recentralized the Shuttle, when the Switch is not preset to allow this to be indicated electrically?... AND the installer having made it not particularly easy for you by installing it upsidedown and maybe in a precarious position for checking, and you don't have a borescope handy?
...I'd suggest checking in a similar way the Switch itself does!...how about checking how far a wooden or plastic (non-damaging!) "depthprobe" (standin for the Switch plunger), is able to be inserted, before the shock inputs, hopefully these will persuade it to move, and the after checks will show a bit less of insertion (I only expect a 1/8 to 3/16" difference, so measure carefully!)...all this, and I hope the internal seals of the Switchblock don't let you down and there is excessive leakage out the Switch port...

Good Hunting!

Thanks to all who helped me with this. I will likely install it with the switch up. To do this I will make a small extension to the bracked so the brake line connections all fit without changing any brake lines. Also I see that there is a small drain hole in the swith. I will make a felt washer to act as a filter to avoid getting crud from abobve into the switch.
Then I should be able to look in and see if it is centered.
I tried to insert a photo here but don't yet know how to do that. I can send a photo if anyone interested.
Thanks again.
Charlie

I think the block is symmetrical? If so you can just take the pipes off and reverse it.

Yes it si symmetrical but the bracket that mounts it to the inner fender is not. I can easily modify the mounting arrangement and reverse it. From the responses earlier it looks like it is upside down in my car and should be installed with the switch up. Thanks.

I'm finally getting back to working on the brake fault valve & switch.

A friend has a spare assembly wo we took it apart to see how it works. Very simple. However, as you all pointed out, it is now clear that the switch will definitley be damaged if one attempts to re-center the shuttle without removing the switch.

It is interesting to note that the shuttle valve is in two parts, two pistons, not one. I am not sure why.

I will install it with the switch facing up so I can see the valve inside. I can avoid modifying any of the brake lines by making a small bracket to install it with the mounting tab to the bottom instead of the top.

Also to avoid getting dirt into the switch I will make a small felt washer to cover the drain hole which will be facing up.

Thanks for your advice on this.

As an observation, if you center the switch operating mechanism (in the non operated position) before installing the switch assembly it will remain centered if you bleed the system using a hand vacuum pump or a pressure bleeder attachment on the MC reservoir. The pressure differentials with vacuum or pressure bleeding are small compared to the hydraulic system operating pressure and will not operate the switch. At least it has never operated the switch on my 142. If you bleed the system using the pedal pump method then the operating mechanism will not remain centered during the bleed process.

...a good observation and Tip by 142Guy!