Residual valves in brake system. Need some education.

Context:

Rebuilding a 1965 Austin Healey 3000 that came from the factory with a single circuit braking system.

The car had disc front and drum rear. 

The disc/drum layout will remain albeit with 4 piston calipers of larger surface area than stock.

It will be getting a dual circuit master cylinder with a front/rear adjustable proportioning valve.

The booster is going away.

It has been suggested that I will need to put residual pressure valves in the circuits. 10 psi for the rear and 2 for the front disc. The car did not have them from the factory, so I am not sure why it would need them now? 

The residual pressure valve is internal to the master cylinder.  A little stamped metal and rubber cup on the outlet end. I have pulled them out of many a vintage British and Italian car, usually when they cause trouble or we upgrade cars from drum to disc up front.

When they go bad on a 4 wheel drum car it will feel like you always need to adjust the brakes. Low pedal that pumps up and holds great but as soon as you let off the pedal for 10 seconds you have to pump it twice again.

I replace them with inline residual valves, usually the Jegs branded ones.

I can't remember doing it on a Healey specifically but all British cars of the era tended to use very similar parts. I'd bet one is in there

I've heard of them used in that application before.  Though I have not experienced it.

My challenge car has them because the reservoir is almost below the bleed screws.  They are seemless in operation and in fact make bleeding easy.  Because they exist in the system you cannot suck air back to the master cylinder.  They are basically a one way valve with a 2 or 10 psi pressure to overcome to return.  I suppose it's possible your return spring will be strong enough to suck them open, but my 2psi ones with a OEM subaru Master cylinder do not do that.  I just pump the pedal until I get solid out a bleeder and that seems to be that.  

I do not have any specs on the master so do not know if they are built in. 

Any harm in putting them in regardless?

I just learned something I did not know but makes so much sense thinking about it. I assume these are used when the master is mounted real low. Like in older cars with the master mounted to a frame rail under the floor board.  

The new calipers could mess with the bias/balance, as could the new master. 
 

You can "turn down" rears but not turn them up, so you'd ideally want to end up with too much rear bias before dialing in the prop valve. 
 

tuned in for Angry's input....

They were a separate valve on TR2-TR4 mounted in line down on the frame, but I don't know if they are available currently. They served the rear circuit.

I don't know that Healeys suffer pad kick back on the front circuit - I certainly never did when I was racing an early TVR using the same calipers.

In reply to wspohn :

The system if far from factory in pretty much all respects.

The only time I have known to use these valves is with hot rods where the master is under the floor. That I can understand the need for.

They are necessary in drum brake systems that do not have springs behind the wheel cylinder pistons.  They hold enough pressure to keep the pistons out and cup seals engaged but not enough to override the springs on the shoes.

I have been known to disable them with an awl when converting to disks.

The thought process at this point is to roll without them.

The car is not mine, and the parts provided were spec'd by the owner according to whatever due diligence he did when he bought the "kit".

If I do put them in, at my suggestion, I worry that I take ownership of these new-fangled  resto-mod brakes that he has created. 

The flip side is that if they are needed, it is pretty much an engine-out job to make new brake lines if they need to be added.

In reply to NOHOME :

You can install them anywhere in the line.  I did a two stage conversion to a '34 Ford and put the residual valve for the rear brakes along the framerail.  (Front brakes were Mustang II disks because of course they were)