Disc brake conversion problem

[dpdmw]

Just a disappointing update. The OEM style disc brake master cylinder with dual ports arrived but would not work as the mounting flange bolt pattern is different than that for the OEM 4-wheel drum. I installed a 2 psi residual valve and re-bled multiple times holding the caliper in all kinds of ways to maximize trapped air to rise and exit and then waited another 24 hours and repeated. I then took the extra step and did the brake pedal bleed method a number of times on both calipers. No improvement I then clamped off the brake hoses going to each caliper as close to the caliper as possible. I get an awesome pedal then. Everything tells me there MUST be air in the caliper but there is simply no other way or process I can use (that I know of) to get it out. I would have easily bet a weeks worth of wages the calipers are completely bled. I have no other straws to grab and I am simply at a complete loss. ANY thoughts would be appreciated.

[Bruce2]

There's no such thing as an "OEM Disc" vs. "OEM Drum" master cylinder. They are all the same. It could be that you received a Std Beetle MC instead of a Super Beetle MC. The flange from the SB MC is rotated about 30º compared to the Std Beetle MC.

Can you post pictures?

[dpdmw]

The disc brake OEM MC was for a 74 model that was standard with front disc and rear drum. At the time it used a different MC and you can still purchase from Rock Auto, specific to front disc/rear drum. The Bentley service manual even refers to it. It was grasping at straws, but I did not have much else. Bolt pattern was at 3 and 9 o'clock, different than the 71 Super I have.

[dpdmw]

Also, '71 Super models in Europe did have front disc as an option, though it did not start in the US until around '74 or so, maybe even a little earlier.

[Bruce2]

Also, '71 Super models in Europe did have front disc as an option, though it did not start in the US until around '74 or so, maybe even a little earlier.

Not one single Beetle sold in North America ever came with disc brakes. It never was an option offered by VWoA. Even in Europe, discs only came on cars with a 1500 or 1600 engine. Since the annual tax on their cars was based on the engine size, most Supers sold in Europe had the smaller 1300cc engine, then came with drums all around. In other words, factory discs in Europe on Supers are rare.

The MC you might buy from RockAuto is the generic disc or drum MC. You can't buy a proper disc brake specific MC like is described in the Bentley manual, no one sells one. The reason is because the generic one works for both discs and drums. I bet you bought the QUALITY-BUILT NM51559 from Rockauto. It's the only one described as for disc brakes, but the photo shows it to be a Std Beetle MC.

If you had a Karmann Ghia in the USA, it did come with discs from VW, and it originally had the disc brake specific MC. But today, all you can get is the generic one. It's been like that for over 40 years.

The 3 & 9 O'Clock MC you got is for a Std Beetle and Karmann Ghia. All Supers have the bolt flange at 1 and 7 O'Clock.

Did you try greasing up your bleeder screws with anti-seize, then manually bleed them?

[dpdmw]

Yes, I coated the bleeder screws with heavy grease. I have bled using a pressure style bleeder and then did the old style press the pedal method on top of that. Just cannot seem to get the air out. Have never run in to this before!

[Ol'fogasaurus]

Try removing the residual pressure valve especially if you are using a dual circuit MC. If the MC has built into it residual pressure valves the addons might be the problem.

Just a thought.

Lee

[Phil69]

I take it you are bleeding the front brakes a side at a time and the pads are firmly on the discs? Also don't forget the peddle will be slightly spongy due to the new pads (also if you've got new linings in the rear drums it will cause a slight spongy feeling too).

[Ol'fogasaurus]

Soft brake pedal which most people agree is usually caused by air in the lines or one of more of the soft lines is dying allowing it to bulge (I have had this happen). Normally the brakes are bled starting in the rear with the cylinder farthest away from the MC cylinder. You get both rear brakes done then you go to the fronts and bleed them starting on the farthest side way from the MC.

Something else I was told about, from a long time ago, just came back into my memory. The dialogue was front brakes are of a larger dia. cylinder and disc brakes cylinder are also larger. If you are running a single cylinder I seem to remember that bleeding the fronts first might be a good idea to try.

I also have heard of putting a shutoff valve at the Tee from the MC to the front brakes and bleeding the fronts isolated from the rear brakes then when all are bled the valve is opened. Again, something I have heard of but never tried myself.

Lee

[slayer61]

FWIW, I had a similar experience bleeding the front brakes on my wife's GPz1100. You would think it was a "no brainer" with the master well above the calipers, but try as I might there was no love from the front brakes. Being "old school" I went back to basics & started from scratch, 2nd checking everything I had done, up to bleeding the front brakes. I found one of the calipers didn't quite sit right on the hangar. Once I got that sorted, It bled out in just a matter of minutes.

20210606_150908.jpg

[SCOTTRODS]

Nice Bike...

[slayer61]

Nice Bike...

Thanks. I'll tell my Mrs. you said so!

[viiking]

Also, '71 Super models in Europe did have front disc as an option, though it did not start in the US until around '74 or so, maybe even a little earlier.

Not one single Beetle sold in North America ever came with disc brakes. It never was an option offered by VWoA. Even in Europe, discs only came on cars with a 1500 or 1600 engine. Since the annual tax on their cars was based on the engine size, most Supers sold in Europe had the smaller 1300cc engine, then came with drums all around. In other words, factory discs in Europe on Supers are rare.

The MC you might buy from RockAuto is the generic disc or drum MC. You can't buy a proper disc brake specific MC like is described in the Bentley manual, no one sells one. The reason is because the generic one works for both discs and drums. I bet you bought the QUALITY-BUILT NM51559 from Rockauto. It's the only one described as for disc brakes, but the photo shows it to be a Std Beetle MC.

If you had a Karmann Ghia in the USA, it did come with discs from VW, and it originally had the disc brake specific MC. But today, all you can get is the generic one. It's been like that for over 40 years.

The 3 & 9 O'Clock MC you got is for a Std Beetle and Karmann Ghia. All Supers have the bolt flange at 1 and 7 O'Clock.

Did you try greasing up your bleeder screws with anti-seize, then manually bleed them?

A bit late to answer but....

I have a RHD 68 with standard VW discs. ATE still make specific VW OEM MC’s!

Re residual pressure valves. These are used on drum brakes to maintain the brake shoes as close to the drum as possible. This reduces the reaction time to apply the brakes. Discs do not require a residual pressure valve as the pads are held close if not touching the disc rotor by the deformation of the O rings in the calliper piston.

And yes given that the closest piston to the driver usually operates the front circuit first and then pushes the secondary rear piston next you need to bleed the fronts first.

[Ol'fogasaurus]

Also, '71 Super models in Europe did have front disc as an option, though it did not start in the US until around '74 or so, maybe even a little earlier.

Not one single Beetle sold in North America ever came with disc brakes. It never was an option offered by VWoA. Even in Europe, discs only came on cars with a 1500 or 1600 engine. Since the annual tax on their cars was based on the engine size, most Supers sold in Europe had the smaller 1300cc engine, then came with drums all around. In other words, factory discs in Europe on Supers are rare.

The MC you might buy from RockAuto is the generic disc or drum MC. You can't buy a proper disc brake specific MC like is described in the Bentley manual, no one sells one. The reason is because the generic one works for both discs and drums. I bet you bought the QUALITY-BUILT NM51559 from Rockauto. It's the only one described as for disc brakes, but the photo shows it to be a Std Beetle MC.

If you had a Karmann Ghia in the USA, it did come with discs from VW, and it originally had the disc brake specific MC. But today, all you can get is the generic one. It's been like that for over 40 years.

The 3 & 9 O'Clock MC you got is for a Std Beetle and Karmann Ghia. All Supers have the bolt flange at 1 and 7 O'Clock.

Did you try greasing up your bleeder screws with anti-seize, then manually bleed them?

A bit late to answer but....

I have a RHD 68 with standard VW discs. ATE still make specific VW OEM MC’s!

Re residual pressure valves. These are used on drum brakes to maintain the brake shoes as close to the drum as possible. This reduces the reaction time to apply the brakes. Discs do not require a residual pressure valve as the pads are held close if not touching the disc rotor by the deformation of the O rings in the calliper piston.

And yes given that the closest piston to the driver usually operates the front circuit first and then pushes the secondary rear piston next you need to bleed the fronts first.

True with both "can" and drum brakes.

Also, when the brakes are applied the weight of the vehicle is transferring forward (unless you are backing up) so the front brakes are usually larger than the rears.

Lee

[OrangeCrusher]

If you can't bleed the brakes you have a bad connection somewhere. Re-examine all your fittings. You're not seeing it because you're too worried about mystery MC's. Your MC isn't gonna make a difference in bleeding. All MC's work the same. Everybody's overthinking it. It cracks me up how desperate people get when they can't get brakes to bleed. It happens to me all the time and every time it's a weak link in the system and I start blaming the boogy-man instead of checking my parts thoroughly. Something is sucking air into your system. Look closely at ALL your fittings and make sure they are symmetrical and clean and the threads are correct and tighten well.
I haven't seen anybody suggest this yet; one trick to use less fluid while bleeding and quicken the process is to unmount the calipers and compress the pistons with "C"-clamps. Leave em on and bleed the system. Less volume to fill in the caliper = quicker bleed. Just remember to add extra fluid to the reservoir after you remount the calipers and before you push the pedal again. This can also be a problem with VW drum brakes that are not adjusted properly before bleeding. The slave can run out of travel and then it just keeps sucking in and out and won't let you get a firm pedal.

Speaking of which; another entry point can possibly be a bad piston oring.

When you've hit your head against the wall this long you need to take a step back and look closer at everything and keep it simple.