New disc brakes work but...

Proportioning valves. You need Proportioning valves. Since the cylinders in the disc's are larger it takes more fluid to move them. Proportioning valves help keep more fluid in the lines to the disc's. You might also consider going to a bus master cylinder.

I hear this complaint often (pedal travel) in my research in buying my own set of disc's.

Ben

My experiences:

I run discs up front in two beetles, one has std beetle calipers, and teh other one uses Talbot Horizon / Dodge Omni calipers. The REAR is too weak! The front locks up very easily, and pedal is solid as a rock and brakes grab VERY early. Very little pedal travel, not bad at all. I have been forced to use "general replacement" master cylinders, seems like the only type available at the time.. and teh seller told me it works well for both drums and discs. WRONG. The disc brake master cylinder has a proportion valve for the REAR cirquit I think, and these masters don't. The threads on the master won't accept the valve either. (different thread size). The proportioning valve would allow the rear shoes to stay as close to the drum as possible slightly rubbing even, leaving a very weak pressure in the wheel cylinders. This way you get brake contact earlier in the rear, right as you start pushing the pedal. Disc brakes are like this by design, they don't need proportioning valves.

I also tried to install FRONT wheel cylinders in the rear, thinking that bigger diameter means more power. Not much difference.. if there is more power, it is counter-acted by the larger volume requiring more pedal travel again. At which point the front discs would be locked anyway.

Jan

when adjusting the rear, make SURE you loosen teh HANDBRAKE cables from teh lever first. Then pump teh brake pedal a few times, then adjust the shoes so that they clearly "drag" half of a full tire rotation. Somehow the contact to drum surface never seems even. There needs to be a slight drag, but the wheel should still turn freely. The worst "drag" will be gone after the first 100 miles.

Jan

Jan,

What's the resolve that works?

I have ghia front calipers/rotors and stock rear drums. peddle travel is good with the stock MC, but yes the rears seem to be soft now. The greater stopping power is wonderful. Once I had to do a panic stop from highway speed and yes I did lock the fronts up so a sec, but handled it. I wouldn't be surprised if my rears aren't doing much work.

Is a bus or ghia MC with a proportioning valve a drop-in to a beetle? The Bently book doesn't mention that these have proportioning valves.

Ok, so now i'm interested in fixing my rear drums so they work better in tandem with my front discs (ghia parts). I want to understand the theory about this.

Should I get the ghia master cylinder for it's different 'restriction drillings' or add a 10 pound residual valve in-line to the rears and keep the stock master cylinder?

I've never seen or heard of a MC that included a proportioning valve. All the cars that I know of use them on the rear circuit only, just to REDUCE the pressure sent to the rear brakes.

Ghias and type 1s with disk brakes don't have proportioning valves, nor do they have MCs that incorporate any special proportioning mechanism.

Ghia and type 1 calipers have 40mm pistons, I'm not sure what dia rear wheel cylinders they use, but this should be a good starting point for you.

What is the piston dia in the Horizon/Omni calipers?

A proportioning valve is nothing more than a hydraulic pressure device. It has nothing to do with the static shoe position. That's simply a function of how you adjust your shoes.

I would have expected this to work, but it might not have been obvious if you were too far from the optimum diameter to start with. Have you looked at a Bentley manual to see what diameter the disk brake type 1s used? I THINK they all use the same 3/4" (19.05mm) MC, so the standard MC should work for you just fine.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Honestly, I don't know! Wilwood adjustable pressure regulator to lower line pressure to both FRONT brakes? Rear would remain stock.. you'd just make teh front weaker.. sounds silly..

Jan

IF you can get the Ghia disc brake master cylinder, THAT is your ticket. Good luck. lemme know if they are available and how it turns out.

Btw: Ghia front discs = std beetle front discs. The master, therefore, should be identical too.

Jan

Will do. As Jim just pointed-out, there's no proportioning valve in the ghia MC, so I think I'll try a combo (metering, prop, rear 10 lbs. residual) along with a 2 lbs. residual on the front together with the stock MC ('73 SB).

Will let you know...

Now to find a wet parking lot and do some pre-testing ;)

Jim,

they are confusing "proportioning valve" and "residual pressure valve".

The disc brake ghias did have residual pressure valves built into the MC, but all replacement MCs do not have this, as they are not needed.

Folks, if your rear brakes are returning so far away from the drums when you don't use the brakes, I suggest you adjust them. The fact that you have discs in the front has NOTHING to do with out-of-adjusted rear brakes.

If you have larger then stock rear tires, and/or smaller (narrower) front tires, you can get more braking balance by putting 19mm wheel cylinders in the REAR (stock is 17mm). Think outside the box; instead of reducing front braking, increase the rear!

John Aircooled.Net Inc.

As far as I'm able to understand it, discs require more volume of fluid while operating compared to drums. That and weight shift forward means you'd want to let off on the rear pressure as pedal pressure increases. That's the job of a proportioning/metering valve.

Just cause the ghia doesn't have a proportioning valve, doesn't it isn't needed. Like 56hp is good enough for 4 people, luggage and climbing the rockies?

PS. My last bug got totaled from a spin-out caused by a panic stop with all around drums for the right front deciding to quit. Now with discs in my current beetle, this isn't going to happen again and I want it fully safe shy of re-inventing ABS.

PPS. Ever driven an ABS equipped car on ice? Where the designers of ABS on happy drugs or did they miss the point of how the car shakes violently during a slide when no wheels have good traction? Just like airbags, I guess there's a downside to everything..

Okay, that makes sense. Jan, I appreciate your position here as an ESL contributor.

I think it was the other way around. Some early drum brake cars got the residual pressure valves. I have some of these RPVs from early MCs if someone wants them. They are old and used, but they have been kept dry, so they have some chance of still working.

In the early days of disk brakes they drilled smaller feed holes to the MC ports leading to the disk brake wheels. I believe this practice was dropped as pointless within a few years.

I had never understood the point of the residual pressure valves. I gather that Jan's description of them was correct: that they serve to make drum brakes somewhat self adjusting. Sounds like wishful thinking to me.

I prefer John's method of just keeping them adjusted myself.

This is certainly the logical course of action. I don't understand why it didn't help in Jan's case.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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I'm afraid you're thinking about it backwards. It's the pressure that is consistent thruout the system, including both front and rear circuits. You won't be able to build up any more pressure in the front circuit than you have in the rear circuit simply because the front circuit pressure is transferred there by the pressure of the rear circuit working on the rear (rear is rear) end of the front circuit piston.

This is for a tandem system, but even with a simplex system it works out the same.

OTOH, you are absolutely right that larger pistons (caliper or drum) will require more MC piston travel.

Your description makes me wonder if your rear wheel flex lines are closed up and only pass fluid under extreme pressure. This would make a kind of extreme proportioning valve and might produce the symptoms you describe.

There is a rather simple test for bad brake flex hoses. First make sure that the passages in your bleed valves are clear; sometimes I find these blocked with rust or dirt. Then open the bleed valve and see if the fluid flows out FREELY just under the influence of gravity.

EACH bleed valve in a healthy system will flow equally freely under gravity. If you have one or more that seem sluggish, then you probably have a flex hose problem.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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They don't allow the wheel cylinder seals to see a vacuum, thus not suck air should they decide to fail in that mode. Also important if your reservoir is lower (gravity) than the calipers to prevent them from draining. Though, not important for us VWAC folks as the reservoir is remote and up on the inside front fender. But true for some custom rods where the MC is under the floor and not a remote type.

I don't think the MC ever delivers a vacuum to the wheel, with or without, the residual pressure valves. Otherwise, why wouldn't cars still use them?

Hmmm, a MC and reservoir that is lower than the wheels it supplies. That sounds pretty bizarre, but it would certainly be a good application for a residual pressure valve. thanks,

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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They do still use them. Most MCs for american cars have them built-in. Wheel cylinders sucking air is a valid fault. It does happen.

"The brakes would not bleed because the shoes (which were still in the OK range) and drums had worn enough to allow the wheel cylinders to suck air (but not leak, go figure)."

He needs residual valves to prevent that from happening again.

I'm getting a headache out of this. What I or you think is irrelevant, if practice shows us it doesn't work.

Not entirely so. The rear can be empty (leak) and the front would still work. Although you'd get more peal travel of course. That's the idea of dual cirquit ;)

Which you DON'T GET if the front locks up as early as it does!!! If I keep pushing the pedal in deeper after the front tires have locked up, then the rear will lock too. This is my point. Wider cylinder bore in the rear requires more fluid to fill it for the piston to travel far enough.

I know this problem well, and I'm no amateur.. :) I have fitted new lines and hoses to both cars.

I like to play it safe, I only use brand new brake components.

BTW, one thing that could help is to replace rear flex hoses with stainless teflon hoses.. the rubber hoses flex and "give" when brakes are applied, not much but enough to make a difference. The ss braided teflon hoses don't. They deliver ALL pressure to the cylinder. So that's what I'll do one of these days.

jan

Yes, that's on my mind, too. I will report the results soon. I know it makes a huge difference on motorcycles for lever feel and would expect the same on a car.