Disc Brake Conversion

[stinger608]

So I did the rear disc brake conversion on my trike a couple of days ago. All went very well except:

After spending two hours bleeding the dang things I still didn't have much brakes. After figuring out that the master cylinder is actually below the level of the calipers I figured I should install a 2lb residual valve. Installed the valve and did yet another bleeding on the brakes. Well, I do have better brakes but it still has to be pumped one time.

What I have figured out is the master cylinder must not be pushing enough fluid on the first pump. The volume is, I figure, not enough. Probably going to need to replace the master to achieve the proper volume of fluid.

I don't want to spend a ton of money on a new cylinder, so:
Wondering what everyone thinks would be the best purchase for a master cylinder?

 

[FuzzyWuzHe]

I would of thought the drum's would need more pressure, but, I've seen many posts talking about the disc's requiring more pressure (volume), so I'm guessing you assessment is correct. Keep plugging big guy ... I'm sure snu will chime in here pretty soon and tell ya just the one to get.

 

[Rsw]

What are you using now?

 

[stinger608]

What are you using now?

Stock single stage VW master cylinder. Same as the ones that were stock on the early to mid 60's bugs.

 

[DeathBySnuSnu]

Typically disc calipers do not require as much volume to engage. The pads retract very little from the disc, especially with a residual valve. Drums must retract a lot farther to disengage fully.
You are usually looking for pressure instead of volume with disc. A lot depends on your mechanical linkage to the master.
I run a 22mm on mine, but I have a large mechanical ratio on my lever.
An early 19mm master should be fine.
Check your brackets for the calipers.
Make sure the caliper centers well on the disc. If it is not near centered it could make it have to retract farther than normal.
Another big problem with the kit brackets is the thickness of the bracket itself. Most if not all of the caliper brackets are way thicker that the drum backing plate by at least .100" When you put these on the bearing/axle/disc and all can float in and out spreading open the caliper. Then next time you hit the pedal it is way down there, one pump and it I'd back where it should be. I lathe cut my caps down. You could use ring shaped spacers between the cap and bearing.

Very few of the kits go over setting them up. I was suprised with mine also. Some of the better kits (expensive) have instructions and a bearing spacer.
Did yours? Was the bracket thicker than the backing plate?
Again... i cut .100 off the bearing cap. End play good. Then put it together and the caliper was not centered. Threw the cut bearing caps in the trash and started over. Spacer under the bearing to get the caliper centered and then cut the remainder off the next set of caps.

Dang long winded today I guess.

 

[stinger608]

Typically disc calipers do not require as much volume to engage. The pads retract very little from the disc, especially with a residual valve. Drums must retract a lot farther to disengage fully.
You are usually looking for pressure instead of volume with disc. A lot depends on your mechanical linkage to the master.
I run a 22mm on mine, but I have a large mechanical ratio on my lever.
An early 19mm master should be fine.
Check your brackets for the calipers.
Make sure the caliper centers well on the disc. If it is not near centered it could make it have to retract farther than normal.
Another big problem with the kit brackets is the thickness of the bracket itself. Most if not all of the caliper brackets are way thicker that the drum backing plate by at least .100" When you put these on the bearing/axle/disc and all can float in and out spreading open the caliper. Then next time you hit the pedal it is way down there, one pump and it I'd back where it should be. I lathe cut my caps down. You could use ring shaped spacers between the cap and bearing.

Very few of the kits go over setting them up. I was suprised with mine also. Some of the better kits (expensive) have instructions and a bearing spacer.
Did yours? Was the bracket thicker than the backing plate?
Again... i cut .100 off the bearing cap. End play good. Then put it together and the caliper was not centered. Threw the cut bearing caps in the trash and started over. Spacer under the bearing to get the caliper centered and then cut the remainder off the next set of caps.

Dang long winded today I guess.

These, which I forgot to mention, are dual piston calipers. Essentially like having 4 wheels with disc brakes.

Yes, they are centered. I had to do some shimming on both sides to ensure that the caliper was centered with the rotor.

As for the bracket, I am not sure how that would effect the braking ability. However, yes the bracket was much thicker than the stock backing plate. To compensate for the difference the kit came with a thick shim/spacer for the rotor to bearing area.

I was reading a thread over at the Samba forums from a fella that had very similar issues after converting a 67 bug to all four wheel single piston disc brakes. The forum thread was about 6 pages long and in the end he installed a 75 Super Beetle master cylinder and it took care of his issues.

Now then, the problem with that scenario is the Super Beetle master is a dual stage master cylinder. I have read on some VW forums that people have just "T"ed into both sides of the master. Now I am not even sure what the heck they are referring to, to be honest.

I took a look Saturday afternoon at a 74 Super Beetle master and sure enough, it has the dual inlets from a remote mounted reservoir making it a dual stage type master. I don't know if I can just use one of the stages on that type of master and just feed fluid in to both of the inlets from a single reservoir or not. I do know it has way more travel than the one I am currently using.

 

[vw driver]

The first image is a dual port MC with a "T". The second is a single port MC used on the older VWs. I used the single port on my trike with the Suzuki dual front disc brakes and the rear VW drum brakes plumbed into it. I got lucky and didn't need a proportioning valve, it stops on a dime. I also retained the cable operated VW emergency brake in case the hydraulic system has a failure.

 

[stinger608]

The first image is a dual port MC with a "T". The second is a single port MC used on the older VWs. I used the single port on my trike with the Suzuki dual front disc brakes and the rear VW drum brakes plumbed into it. I got lucky and didn't need a proportioning valve, it stops on a dime. I also retained the cable operated VW emergency brake in case the hydraulic system has a failure.
View attachment 29010

Thanks Driver! Yea the second (lower) picture is what I'm currently using and from the research that I have done it doesn't have enough volume to properly operate the dual piston calipers. I have read that people have gone to the dual stage master like the top picture in your example.
Then they used a "T" to give it more volume. Of course the "T" will only work running a trike scenario.

Buddy of mine's older Harley trike we plumed dual disc front and disc rear into a 7/8" bore master cylinder and it works great.

LOL, this project of mine has been a bit frustrating though. First I figured just putting a residual valve in would work due to the master being lower than the calipers but I still have to give it one pump. Then I have fantastic brakes on the second push.

 

[DeathBySnuSnu]

At one point in the past I had my 12mm bore handlebar lever master cylinder hooked to my back disc brakes.
It just almost worked, just a little shy on displacement.
I have seen as small as a 17mm work well. My 22mm is a bit stiff, you have to want to skid the tires.
What calipers do you have?

 

[stinger608]

The kit that I purchased was EMPI 22-2861. It is the bottom kit on this page:

http://vwcatalog.empius.com/vwcatalog/2014/183.html

 

[DeathBySnuSnu]

Same stuff as mine.
I got the non drilled and the cast brackets.
I'm using a 22mm master on mine. I have the original motorcycle pedal and It is sorta long. With where my foot pegs are I can not roll my ankle over as far as a normal bike sitting position can.
Combining these two things I need a quicker response or less travel until pressured up.
My brake pedal don't move very far at all compared to the original car or the amount of stroke available inside the cylinder. My brake pedal is fairly stiff, not too much, I can skid it if I want to, and have skidded it in a panic before. I do wish the pedal was not quite as stiff sometimes like with a 19mm master, but I can't afford the pedal travel with my feet so far forward.
I really don't understand how anything more than a 17mm master would have to be pumped to get the volume needed. It still leaves me thinking something else is wrong, unless the current master is going out anyway.
What size bore does the super beetle master have? Of course only one piston works the disc. Teeing them together just gives you more piston area/volume/displacement going to one place. Would be the same as putting a really big single master on.

 

[stinger608]

Hmm, man, I suppose the master could be going bad. It was installed new about a year ago but of course that doesn't mean squat. :laugh:

 

[vw driver]

Thanks Driver! Yea the second (lower) picture is what I'm currently using and from the research that I have done it doesn't have enough volume to properly operate the dual piston calipers. I have read that people have gone to the dual stage master like the top picture in your example.
Then they used a "T" to give it more volume. Of course the "T" will only work running a trike scenario.

Buddy of mine's older Harley trike we plumed dual disc front and disc rear into a 7/8" bore master cylinder and it works great.

LOL, this project of mine has been a bit frustrating though. First I figured just putting a residual valve in would work due to the master being lower than the calipers but I still have to give it one pump. Then I have fantastic brakes on the second push.

I should have described then as single chamber/circuit and dual chamber/circuit MCs rather than single and dual port. My MC is lower than the front calipers and the rear wheel cylinders and has not created any problems. It's a stock VW single circuit with a 17mm bore which works out to be 43/64" which is just shy of 11/16". A 7/8" MC would be 22.2250 mm so it's quite a bit larger. It sounds like you have the right combination it's just a matter of eliminating the one pump. I'd try bleeding it some more and see if that gets rid of the pumping.

 

[DeathBySnuSnu]

Did ya research and find the bore size of a super with disc brakes?

 

[stinger608]

Did ya research and find the bore size of a super with disc brakes?

I haven't found that out yet, no.

 

[MdKnightMan]

Simple How To's

Hi, As a relative noob, I don't have the tech expertise to help with this, but the thread was titled Disc Brake Conversion and I wanted to add something here for future researchers. When I decided to add disk brakes to my trike project I went looking for GOOD tutorials on this. However many times, I would find an old link on samba or others saying "here is a great tutorial" only to find the link inactive. For me, "good" means step by step with explanations and close up pictures. Details of problems and what was done to overcome them. In this vein, I have found 3 posts, out of a hundred I looked at, that did this. Much thanks to the original posters (I would reproduce them here if I new how, if someone else does, I have them copied as both pdfs and word docs so they are here in case they too disappear). Again, I give all credit to the original posters. For an awesome tutorial on a "CB Performance rear disc kit" I found this
http://www.huelsmann.us/bugman/Rear_Disc_Tech.html

On alternative Rear brake solutions I found these; using Golf/ Passat calipers with Porsche 914 rotors and turned VW drums.
http://www.gerrelt.nl/section-modifications/other-modifications-rear-disks.html?showall=1

and using Ford Sierra Cosworth Calipers also with Porsche 914 rotors and turned VW drums.
http://www.airsouls.com/how-tos/rear_discs.htm

worth mentioning but not as detailed is this one using 79-84 Datsun/Nissan 280ZX Calipers.
http://www.blindchickenracing.com/How_to/280Zbrakes/280zbrakes.htm