Brakes stuck on

[Series1 Stu]

The same thing happened on my 420, which has pretty much the same braking system as the Series 2 E Type. It was the reaction valve on the front of the master cylinder, supplied by you know who, that had been assembled with the piston the wrong way around and therefore maintaining vacuum on the brake circuits. A tiny little plastic part, capable of freezing a 1.5 Ton car.

Regards

[Monkeyfinger]

I have carried out some more diagnostic work today - I popped the vacuum hose to the front of the servo, off at the bulkhead connection point.

With my hand brake bleeder vacuum pump connected to the servo front via that same hose, I am unable to pull any vacuum. That would be the servo assistance side.
However, I can pull a vacuum using the same approach on the atmospheric pressure side.

So that would tell me that the servo diaphragm is ok, but that potentially I have a leak on the vacuum side. I know there are some seals/gaskets in there.

Now, as I'm not able to drive the car, I can't immediately tell if there is any servo assistance, and I'm also unsure why this would lock the brakes in any case, but it can't be good. I know I can do some tests starting the engine with the pedal depressed, so I'll get on to that.

I will have a good look at the brake circuit schematic and see if I can figure what the heck is going on.
Any input most welcome!

[mgcjag]

You need to confirm do the brakes lock.....and only release when you remove the vacume?.....then narrow down why is the vacume not releasing....Steve

[Monkeyfinger]

Ok, some more checks -

Started the engine with foot on the brake pedal - no downward movement of the brake pedal, and obviously no servo operation.

I've been mucking about with the reaction valve and replaced some of the vac pipes on it and the check valve, all connections tight and no signs of leakage there. Since I already tried unsuccessfully pulling a vacuum on the servo directly, I think safe to say there's a problem there.

Also, no amount of pedal pushing is causing the brakes to lock on now.

So I plan to get a front end kit for the reaction valve (cheap), and a complete new servo/slave cylinder (not so cheap).

I still can't really compute how this may have resulted in brake lock on, but it seems the MC piston is operating ok, and I (probably) don't have an issue in that area.

[mgcjag]

Hi Richard....dont just assume that new after.arlet parts work out of the box...we have had members here fit replacent servo,s than are faulty....my guess would be your reaction was sticking and locking your brakes...dismantling has un stuck it.....always the possibity of a servo fault as well....Steve

[Monkeyfinger]

Absolutely agree on the state of replacement parts - current servo is not holding vacuum, so I guess a leak on the join between the slave and the servo body. I will certainly test this before installing a new one. I'm just not keen to simply try and fit a repair kit, so a replacement is my favoured option to at least (hopefully) know I'm starting with a good working unit.

The operation of the reaction valve remains a mystery to me. For something that appears simple, it seems to achieve a complex job with activating and modulating the servo, with some rubber and plastic that doesn't look like it will seal properly, and yet it does.

[Tommd]

Richard, here is a write up on how the reaction valve works if you are interested:

https://forums.jag-lovers.com/t/brake-i ... /371019/36

When you say you replace the reaction valve, what all did you replace. The "kits" that I have seen do not include the poppet, #8 in SNG's parts breakdown. I have actually seen this part missing from some, but that creates no boost, not locked brakes.
Tom

[Monkeyfinger]

Thanks Tom, that's one of the many RV forum discussions I have read!

I should be getting the entire nose of the valve and the diaphragm. It will allow me to compare between my 'original' RV (possibly from build date , and marked Lockheed), the one on my relatively recent replacement MC (also marked Lockheed), and this new pattern part.

[Monkeyfinger]

Whilst I undergo replacement of the servo/slave assembly, I've compared the different RV's that I now have, with the pattern part I've purchased.

There is one significant difference, and that is the spring in the nose of the valve. Attached image shows a comparison. The new (pattern) RV spring is much longer and stronger than the Girling version. A very non-scientific test shows the preloaded Girling spring at no more than 300g, and the pattern part at at least 800g of load.

It remains to be seen how this will work in reality, but there's clearly going to be a much higher return push on the diaphragm plunger, transferring to the MC small piston. I shall report back when I've managed to get the servo back in and start testing.