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1974 412 Brake Issues
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turboblue
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Joined: October 09, 2003
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Location: Central Indiana
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PostPosted: Fri Jun 11, 2021 2:02 pm    Post subject: 1974 412 Brake Issues Reply with quote

Hello

Haeving brake issues with a car we're trying to resurrect.
Ben sitting since 1991.
Th master cylinder, calipers, rear brake cylinders and the 4 hoses have been replaced.
This is part of the rear braking system.
Can't get anything past this.

What is it's function?

Thanks in advance.

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Turbo VW Sand Drag Buggy

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raygreenwood
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PostPosted: Fri Jun 11, 2021 5:12 pm    Post subject: Re: 1974 412 Brake Issues Reply with quote

So....this means you have either a two door or four door because wagons do not have that part.

That is the rear brake proportioning valve and pressure regulator . Its a very cool piece of equipment.

What it does......it is kind of like an early anti-lock brake system. It prevents wheel lock up during very hard or panic braking.....to prevent rear wheel skid .....which is prone to letting the rear of the car come unglued from the road and wanting to pass the front end.

As you stomp on the brakes hard.....with any car.....the front end dives and the rear end lifts. But because all of our heavy weight....engine and transmission.....are in the rear.....excessive weight shifts upward and forward. This takes traction weight off the rear wheels.....so as the pressure keeps increasing during a panic brake.....the wheels in the rear lock up.

What this unit does......is it allows brake pressure to flow to the rear brakes until it reaches 523 psi.....then it locks the pressure so it cannot increase any higher.....but keeps that pressure on.....until you release the pedal and then it backs down at a normal rate.

It is a differential pressure regulator.

It has a 15mm bore with a single rubber cup on a spool valve....with a poppet valve on one end and an adjustable coil spring on the other.

You cannot buy a new one.....but.....this same part was used on Porsche 914, some BMW and some Mercedes.....so PMB performance....makes two options for it.

1. They make a kit with all new seals and o-rings.....but you will be responsible for cylinder refurbishment and recalibration.

With the amount of age delated rust I have found in these things they take a bit of work. The seal is no picnic to install...even though they send you a mandrel in the kit. Adjusting it means you need to buy a cheap 600 psi gauge.

The rebuild kit is $89. A gauge would cost you maybe $35 at Mcmmaster carr....a tee, a tube and fittings.....maybe $135 with shipping.

2. PMB performance will take your unit...fully rebuild it, re-plate the outside with zinc and paint it and calibrate it.....$179.

https://m.pmbperformance.com/page/subpage/914-brakes/Early_Pressure_Regulator.htm

I will post my page on these later so you can see how they work.

I have about a half dozen I will be replating the inside of the bore for along with master and slave cylinders hopefully in July as usable cores.

Ray
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raygreenwood
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PostPosted: Fri Jun 11, 2021 5:23 pm    Post subject: Re: 1974 412 Brake Issues Reply with quote

Here i my page on these units

https://www.thesamba.com/vw/forum/viewtopic.php?t=664753

Ray
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reluctantartist
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PostPosted: Thu Aug 26, 2021 3:23 pm    Post subject: Re: 1974 412 Brake Issues Reply with quote

How hard would it be to put one of these on a 412 wagon? I have experienced the rear wheel lock up especially immediately after it rains.
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1982 Westy, 1974 412 Variant... Yes, Aircooled's are great! Oh and I do have modern computer controlled vehicles too, but I just don't care about them.
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raygreenwood
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PostPosted: Thu Aug 26, 2021 7:26 pm    Post subject: Re: 1974 412 Brake Issues Reply with quote

reluctantartist wrote:
How hard would it be to put one of these on a 412 wagon? I have experienced the rear wheel lock up especially immediately after it rains.



Ah!

Never had that on any of the wagons I owned...and was probably lucky. I cannot understand why they never installed them. But since the weight and wheelbase are roughly the same...there is no reason why it would not need one or could not benefit from one.

The wagon also came without the rear sway bar, but all of the mounting holes and threads are there. Yu can install a sway bar from a sedan on the wagon in roughly 30 minutes with nothing but a handful of wrenches.

And....thinking about it.....it may have something to do with the swaybar and trailing wishbone rotation.... with why they thought they did not need a rer brae regulator.

So installing one is as simple as bolting or tack welding a bracket for the regulator on and a couple of simple bent pieces of brake line. If memory serves...on the wagon...there is a TEE right there to feed both rear wheels. So with a little light bending and reshaping of the lines and maybe a union...you can install this and bleed teh brakes and you are done.

You can have the valve fully rebuilt by PMB performance...if you can find a core.
Bear in mind...at my links...that most of those who look for and sell these valves are selling one from a Porsche 914 or a BMW. So the prices may be stupid.

Some of the BMW units only have one "out" port but have the bosses for the extra one we need. You can either drill and tap that...or simply put a brass TEE on the single port and plumb it to both rear wheels.

There is an untested one on sale on ebay from the UK for about $85 pounds.....thats not a bad price. Ray
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