10 replies to this topic

[MartinML]

Hello,

i have 74 Mini with GMC159 master cylinder and PDWA valve. All 4 drumm brakes. On the rear frame I have inertia valve 13H7757.

 

I changed front drum brakes to 7.5 disc brakes, I replaced the non-functioning GMC 159 with GMC227.

Now I need replace the non-functioning PDWA valve (on photo).

I've been told it's pointless to put FAM7821 when I already have inertia valve 13H7757 in the back.

 

So what part to use instead of PDWA?

 

Thank you

Martin

 

Attached Files

•  PDWA.jpg   94.61K   2 downloads

[gaspen]

I have a similar setup : GMC227 MC and 13H7757 on the rear subframe, but mine has drum all around. I learnt on this forum that 13H7757 "load sensing" valve is the best and simplest option.

 

I just discraded the PDWA* from the bulkhead and made the brake lines directly from the MC to the front and rear.

 

*That PDWA valve on your picture is no longer available. If you want to sell it you can get good money for it.

 

 

 IMG_20210703_141947.jpg   74.21K   13 downloads

[cal844]

Just make new brake lines from the master cylinder to the 2 pipes that exited the pdwa

[Ethel]

A yellow band master (stepped bore) would constantly trip the PDWA so it's best removed & replaced with a float switch reservoir cap. As said, on the hydraulic side you could make up new lines, or use a straight connector.

[Itsaminithing]

It would have been nice to keep the original PDWA valve.... but not a good idea if it causes problems with that new master cylinder!

 

For anyone else reading this thread who has a 13H5905 PDWA valve note that new ones are still available-

https://www.somerfor...witch-74-80-lhd

 

or you can rebuild old ones-

http://www.74tr6.com/pdwa.htm

 

[gaspen]

It would have been nice to keep the original PDWA valve.... but not a good idea if it causes problems with that new master cylinder!

 

For anyone else reading this thread who has a 13H5905 PDWA valve note that new ones are still available-

https://www.somerfor...witch-74-80-lhd

 

or you can rebuild old ones-

http://www.74tr6.com/pdwa.htm

 

Did they sell one for that price?

[Spider]

The purpose of the PDWA Valve is only to provide a means of tripping a switch to let you know that there's been a failure of one of the Brake Circuits. It does nothing else and as far as Braking Performance goes, removing it will not adversely affect the Brakes in any way, if anything they may feel a bit firmer with the removal of rubber from the system.

To still provide indication of a Failure of one of the circuits, in the lid of the GNC227 there is a float switch, you can use that and it's MUCH more reliable than the PDWA Valve ever was.

 

Be aware though, that the lines for the GMC227 are swapped around, ie the top port is connected to the front brakes. This is important as these master cylinders have a stepped bore in them.

[Maccmike8]

The purpose of the PDWA Valve is only to provide a means of tripping a switch to let you know that there's been a failure of one of the Brake Circuits. It does nothing else and as far as Braking Performance goes, removing it will not adversely affect the Brakes in any way, 

 

Does that count on later bulk head mounted valves too?

[Ethel]

If you mean the FAM7821, no that's a pressure limiter. It stops the rear wheels locking up if you brake hard. It does more than the one that was mounted on the rear subframe with single circuit brakes. It's operated by the front brake circuit, so if that fails the rear brakes won't be limited. If the rear circuit fails it'll close it off so you don't lose all the brake fluid.

[Spider]

 

The purpose of the PDWA Valve is only to provide a means of tripping a switch to let you know that there's been a failure of one of the Brake Circuits. It does nothing else and as far as Braking Performance goes, removing it will not adversely affect the Brakes in any way, 

 

Does that count on later bulk head mounted valves too?

 

 

No.

Those are there for to limit the maximum pressure to the rear brakes. The OP here has an Inertia Valve fitted.

<Edit: Ethel beat me to it and with a more comprehensive explanation >

Just about all Brake set up on Mini need some sort of pressure control to the rear brakes to stop them locking, though there were some that didn't have any valve, but used smaller rear wheel cylinders, typically 1/2".

Generally, the set ups that don't use a PDWA have a switch on the master cylinder. Some had them in the body of the cylinder - which is actually a built in PDWA Valve - and others had a float switch in the cap, which I've found to be very reliable.

 

Cut-away diagram of a GMC167 Master Cylinder ;-

 

The PDWA Valve is the bit that's shown in the smaller drawing.

Edited by Spider, 20 November 2021 - 12:05 AM.

[Maccmike8]

Thanks folk.