Originally posted on iso-100.com (domain no longer active) on October 13th, 2012. Copied here for posterity and to support the community. cg
REMOVING THE REAR BRAKE BIAS VALVE FROM YOUR XR4TI
About a year ago I sold my Volkswagen Passat and purchased a 1988 Merkur XR4Ti. While its stock braking system was likely pretty advanced for the eighties, it wasn't good for the increased performance levels I was aiming to achieve with the car.
Stock, the XR4Ti gets about 175 horsepower from the 2.3l turbo engine. In my current state of tune, I’m closer to 290hp and 350ft/lb of torque. With this much of an upgrade to power, I needed a similar upgrade to the braking system.
Stock Braking System
From the factory, the XR4Ti came with 10.2” front ventilated discs and drums in the back. Again, this is great for the 80’s, especially for a car that weighs around 2800lbs.
Rear Proportioning/Bias Valve
Mounted under the master cylinder up front is a seemingly odd contraption. It’s hooked up to only the rear braking circuit and is bolted to the driver-side engine compartment wall.
Inside is a 1 inch stainless steel ball on about a 45 degree incline. Under heavy deceleration, this ball rolls uphill and reduces the brake pressure to the rear circuit.
Why Limit Pressure to the Rear Brakes?
The rear drum brakes on the Merkur are 1/2 self-servoing which means that the drag on the applied brake shoe causes one of the two shoes to wedge itself into the drum lining. This works a bit like a brake booster, increasing braking effect without additional input from the driver.
Discs are inherently non self-servoing, relying purely on piston pressure for braking force. The lack of self-servoing action is why disc brake equipped cars pretty much all have power brakes (brake boosters). Without the help a lot of pedal pressure would be required.
While line pressure rises equally front and rear, the braking force of the drums is rising faster than the discs. This situation is exacerbated by weight transfer to the front of the car. Inertia acting on the ball/ramp in the valve locks the rear line pressure at a level predetermined to be below that causing lockup with any increased weight transfer. This assumptions is fairly conservative and thus the rear brakes tend to be cut off at a very low braking force under firm to hard braking.
Why ditch the valve?
With disc brakes now on the front and rear of my car, it no longer needs to limit braking pressure to the rear. The rear brakes are discs and don’t have one shoe on each side increasing braking pressure. This valve is now counter productive. In my case it was both leaking (why does it have a weep valve??) and had air bubbles inside, making my brake pedal feel like a marshmallow
I hemmed and hawed about getting the right parts up front before tearing apart my braking system… mainly because my XR is my daily driver. I wanted to have all the parts ready to go so that when I started the job I could finish it. I did a lot of research online at Merkursport and Merkur Club but there still wasn’t a definitive answer to the size of the lines that go into the valve. Some said it was M12x1, others said M10x1 and most even said the seat was an inverted flare.
I looked all over the internet looking for a female M12x1 coupler with an inverted flare seat… but all I could find was a bubble seat.
It turns out that the bubble seat version is what’s needed. Just go to your local auto parts store and pick one up for $2.49. Here’s [what] Advance Autoparts online store [calls it]: Brass Brake Line Union - 1 per package - 6 mm (M12 x 1.0 Bubble)
Now, just remove both brake lines from the valve using a 13mm wrench. Then remove the bolt from the side (14mm I think) to remove the valve from the car. I carefully bent the two lines into close proximity of eachother and installed the brass union in-between. Bleed the rear brakes and you’re done. $2.50 and 20m of work.
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