Brake Pressure Differential Valves

[papajam]

I thought to post this thread to explain the purpose and function of Brake Pressure Differential Valves (BPDV).
The BPDV does what it's name implies; it reacts to a difference in brake pressure between the front and rear brake circuits on vehicles equipped with tandem brakes. The BPDV's sole purpose is to turn on a warning light on the dash if it senses a pressure difference between the two brake circuits.
A BPDV is essentually a block of metal that has provisions for the attachment of four brakelines and an electric switch. Internally, there is a piston (or pistons) inside a machined bore, springs and piston seals. The springs help the pistons to self-center. The normally open (NO) switch has a plunger that rests in a recess between the two pistons.
Pic 1 shows a BPDV from a 1969 US version 1750. This BPDV is unique in that it has the two hydraulic brakelight pressure switches which are mounted to the valve only as a matter of convenience; the switches have nothing to do with the operation of the BPDV. When there is zero pressure in the system (brakes released), the pistons are centered in the bore. When the brakes are applied, there is equal pressure on both sides of the pistons so the pistons remain centered.
If, when the brakes are applied, there is more pressure on one piston than the other piston (pic 2), the piston will move a few millimeters to the side that has less pressure. When this happens, the angle on the piston (pic 3) pushes on the switch plunger. This closes the switch which completes the ground circuit for the brake warning light on the dash turning on the light.

There is a misconception that the BPDV closes off brakefluid flow if there is a hydraulic failure in one circuit causing the BPDV pistons to move to one side. This is not the case as evidenced by the physical dimensions of the valve and it components. The diameter of the large portion of the piston is about 9.4mm. The small portion is about 5.6mm. The cavities inbetween the input and output lines is about 11.2mm. A 5.6mm piston will not shut off fluid flow in an 11.2mm cavity.

[60sRacer]

Thanks Jim

This valve was used on the 1750's, with dual Bendix boosters I think.

Later cars with dual brakes have the booster before the MC, and a dual circuit MC. I've found that this is the culprit when I had problems bleeding the brake circuits. The dual MC has a floating piston between the two circuits that needs to be centered to bleed the brakes - hence the factory direction on these later cars to bleed a right front and left rear (or v-v) simultaneously.

A related problem occurs in the rear brake pressure limiting valve, which can close or not depending on how you operate the pedal when bleeding.

Vacuum bleeders are suposed to make all this a lot easier. Personally I've had no better success with one of these. Also tried a pressure bleeder (a factory tool for my BMW) with only slightly better success.

Mostly I found that going slowly and having a lot of patience - and a lot of spare brake fluid - is what it takes.

Robert

[superloaf]

Quote:

Originally Posted by papajam There is a misconception that the BPDV closes off brakefluid flow if there is a hydraulic failure in one circuit causing the BPDV pistons to move to one side. This is not the case as evidenced by the physical dimensions of the valve and it components. The diameter of the large portion of the piston is about 9.4mm. The small portion is about 5.6mm. The cavities inbetween the input and output lines is about 11.2mm. A 5.6mm piston will not shut off fluid flow in an 11.2mm cavity.

[superloaf]

later valve pictures (complete with internals) posted on my thread with the link above.

[MidLifeCrisis]

Jim,

Please come to Connecticut... I owe you dinner for this post

Michael.

[velooce]

On my 69 vintage race car with stock alfa brakes with the boosters removed has started to develop more rear brake bais. I would replace the front pads 2 times for every one set in the rear and now its about 1 for 1. I have had no problem bleeding the brakes. I am suspecting that the rear Brake Portioning Valve is failing and needs to be rebuilt or repaced. Is this valve unique to 69 or is 69 - 74 the same since the calipers are the same. Or could the brake pressure differential valves cause this symtom ? I would appreciate your opinions.
John

[60sRacer]

For a racer I'd use one opf the adjustable rear valves in place of the stock one. Tilton makes a good one. Cheaper than finding a rebuild kit I think, and allows you to adjsut rear brake proportion during the race if you locate it well. One version is 7 stop lever, the other is a twist knob.

http://www.tiltonracing.com/content....ist2&id=36&m=b

Robert

[papajam]

Quote:

Originally Posted by 60sRacer The dual MC has a floating piston between the two circuits that needs to be centered to bleed the brakes - hence the factory direction on these later cars to bleed a right front and left rear (or v-v) simultaneously.

[papajam]

velooce,
The rear brake limiting valve is common to many Alfas with tandem brakes for many years. There is a difference though in that the early valves, 1968-mid 70s (I think) were adjustable. The later valves are non-adjustable. My understanding is that only the later style are availble.
I'm with Robert on this. Since you track the car, I'd go with an aftermarket adjustable.
And no, the BPDV would not be the cause.

[velooce]

Thank you - Jim and Robert

Have you ever tried adjusting this valve, the directions suggest not but it seems that spring tension could weaken after 38years ? Most of the after market are sae threads or for a balance bar set up and since the Alfa brakes are quite adaqute adjusting or replacing is seems a lot simpler.

John

[60sRacer]

The tilton valve is available in both SAE and metric, and in a knob adjustable and a lever with seven steps I think. HEre's teh web:


http://www.tiltonracing.com/content....ist2&id=38&m=b


I've used the lever adjustable as it's repeatable, and have even adjusted it in a race as the fuel is used or the track changes.

Robert