Brake proportioning valve adjustment

ERAUGTV · #1 (**permalink**) 03-03-2005, 09:19 PM

Is there any way to adjust the proportioning valve? My rear tires lock up before my fronts which makes things pretty interesting. I have a new MC and the system is blead well. I also have slotted rotors on the front and regular on the back, but I don't think that the difference in surface area is enough to matter. The car in question is a '74 GTV.

Thanks,
Jason

**Trav** · #2 (**permalink**) 03-04-2005, 02:30 PM

The stock valve on my 71 appears to be adjusable, at least it has a threaded adjuster-looking-screw coming out of it. I never messed with it. The replacements do not have an adjuster-looking-screw.

Refering to Papajams' response to the previous thread on brake adjusters it stands to reason yours may be bad. Papajam discribes its function as pressure reducer. A broken pressure reducer would allow too much pressure and result in premature locking.

I don't know how to check if its working or not. New ones are available and not too expensive.

TD

**papajam** · #3 (**permalink**) 03-04-2005, 05:28 PM

I didn't forget you, Jason. I haven't yet found the document that explains the procedure.

But it's coming.....

JoeCab · #4 (**permalink**) 03-04-2005, 05:40 PM

Jason,

As far as I know, mid-year '72 was the end of the adjustable proportioning valve. Even if it were adjustable, by this time the problem is really not the adjustment per se but the amount of rust/sediment/gunk built up in there. I would advise replacement - with either stock Alfa unit or a nice Tilton adjustable unit which is readily available. I have never seen a proportioning valve pulled of a car of this vintage that didn't look horrible.

Joe

**alfaparticle** · #5 (**permalink**) 03-04-2005, 07:13 PM

I had the same problem with my 73 GTV after I replaced the rear pads but not the front ones. I then replaced the front pads with similar ones to the those in the back and I had no more problems. In fact when I wrecked the car I left four distinct trails of rubber on the road and flat spotted all four tires.
Ed Prytherch

**papajam** · #6 (**permalink**) 03-04-2005, 07:41 PM

Found the document!!

But first, Ed brings up a very good point. Perhaps the problem is not too much pressure to the rear but rather not enough pressure to the front. Front brake hoses, master cylinder, pressure differential valve, different brand tires front and rear and weight transfer could also be possible causes.

This three page document explaining the operating principles of the valve is a bit too much to post here but you PM me your email address, I'll scan and send it to you. If it does turn out to be the valve, I'd do as Joe suggests and replace it.

JoeCab · #7 (**permalink**) 03-05-2005, 07:30 PM

If it helps, here are some pictures of the '72-on non-adjustable proportioning valve. These were also used on Spiders through 1994 and GTV/6s 1981-84. We keep them in stock at Centerline, part no. BC455 currently $114.00.

Joe

**kengta** · #8 (**permalink**) 03-14-2005, 10:20 AM

On 1 or 2 GT club racers from the 60's and 70's that I have owned, the Alfa stock brake prportioning valve was mounted in the passenger compartment with an allen key brazed into the adjusting screw so the driver could do his adjustment at will. Club racers then were often a frugal lot.

Ken Geiger
Toronto

**papajam** · #9 (**permalink**) 03-14-2005, 11:07 AM

Hey Ken,
Which direction would you turn the screw to reduce the pressure to the rears?

**kengta** · #10 (**permalink**) 03-16-2005, 06:33 AM

Jim, I don't know from personal experience but after looking at the Tech bulletin you sent me, thanks, it seems to me that if you turned out the adjust screw(counter clockwise) you would reduce the force of the spring on the piston and thereby allowing it, the piston, to move under less pressure, sooner, thereby modulating the pressure to the rear sooner at a lower pressure.

Regards

Ken Geiger
Toronto

**papajam** · #11 (**permalink**) 03-16-2005, 10:06 AM

Yeah, that's my thought too.

**kengta** · #12 (**permalink**) 03-16-2005, 02:08 PM

BTW, I have disassembled one of the 105 units , cleaned it up and reassembled. worked fine after.

ken

chasking · #13 (**permalink**) 04-26-2006, 01:28 PM

I'm resurrecting this thread because last night my restoration work on my '71 GTV involved the brake system, and I'm wondering what to do regarding the proportioning valve. It's the adjustable kind, and when I took it off the car it appeared to be a solid chunk of rust. But, a few passes with a wire brush and the unit underneath began to emerge. It's an ATE unit, and it looks to be pretty solid, although I don't know whether or how it works. New ones are pretty expensive, and I don't know if they are the adjustable ones---I suspect the ones offered by the usual vendors are like the one pictured above from Centerline.

So: do these things go bad, and if so, in what way? If possible I'd like to save this one, but I certainly don't want to do anything that would put the integrity of the brake system at risk.

I am greatly heartened by Ken's post, that they can be disassembled, cleaned up and reassembled.. Ken, if you're still out there, do you have any more words of wisdom? Or anyone else who has done the same thing?