4 Wheel Discs - New master cylinder?

[jeepsterryan]

I've read through multiple threads and can't find a definitive answer. If you don't want to read the whole story, I just need to know if a larger master cylinder is required when upgrading to disc brakes on all 4 wheels.

The long story is this. We bought a 69/71 baja that already had disc brakes on the front. Included in the purchase were 3x3 extended trailing arms and disc brakes for the rear. One of the rear drums had a rattling sound, was locking up, and obviously had a problem. Upon disassembly, the brakes hardware had come loose and was destroyed. But, looking at the disc brakes and rotors that had come with the car, they looked like a simple bolt-on installation. So, we put them on and started bleeding the brakes.

During the bleed process, the calibers would build pressure and bleed, but the pressure was not enough to stop me from being able to rotate the rotors with a wrench on the stub axle nut with relatively little force. I went to check the fluid level in the master cylinder and noticed a huge puddle of fluid at the front of the car. Long story short, one of the front Jamar calipers had no o-rings between the two halves of the caliper. The previous owner had just sealed the caliper by tightening the bolts as much as possible and securing them with a ridiculous amount of red Loctite

I installed new o-rings and re-installed the front calipers. I removed the rear calipers and made sure the pistons were not bound up and then reinstalled the calipers and bled the brakes. However, now the brakes will not stop the vehicle. The pedal moves about 1/2 through its range of motion before it feels like the master cylinder is engaging. With the drums on, the brakes worked great (even with one rear drum malfunctioning). Now I can't even stop the car. For the sake of helping me diagnose the problem, I only have a single brake light switch installed on the caliper. It is in the rearmost port and it lights up right as the pedal engages the master cylinder, but only briefly, and it won't stay lit as the pedal is depressed (it is new and was working with the original disc in front, drums rear set up).

Because I have so little experience with the beetle parts and how they work, I'm not sure which course of action to take, but I think it is one of these 3 things:

1) The master cylinder must be upgraded when adding disc brakes to all 4 wheels - I think this may be the problem, but the fact that the master cylinder doesn't build any pressure until 1/2 way through the stroke, and the brake light switch won't work, makes me think it is some other problem.

2) When we were bleeding the rear brakes, the front reservoir ran all the way dry and we introduced air into the master cylinder - I could bench bleed the master cylinder, or pay to have the system vacuum bled, but I don't want to do that if the problem is actually that I just need a new, larger master cylinder.

3) It is some other problem that the VW Gurus here know about that I don't know about because I'm just a rookie:)

Any thoughts?


This is my master cylinder: http://vwparts.aircooled.net/Brake-Master-Cylinder-Brazilian-1967-Std-Beetle-p/113-611-015bdbr.htm
These are my front Jamar brakes: https://www.mooreparts.com/jamar-performance-silver-2-piston-caliper-includes-brake-pads/
These are my rear brakes: https://www.racereadyproducts.com/cnc-calipers/cnc-series-626-two-piston-caliper-caliper/

[Pam's buggy]

I am not a VW guru.
But I have worked on many cars and bikes.

What you need to consider bore and stroke of the old wheel cylinder and the bore and stroke of the caliper.

If the bore is larger on the caliper,it will take more brake fluid to move the Piston.

If it's a duel Piston caliper measure both Piston.
Take the wheel cylinder apart and check the bore. Do the same with the caliper.

You will need a larger master cylinder if there is a difference.
Hope that helps

[oldschool5er]

Your stock master cylinder will not work with Jamar front brakes with the CNC rears. You will need to go with a larger bore master cylinder. I have always had to move away from a stock dual chamber master when using Jamar and CNC. Give Jamar a call as they can recommend the right master from them, make sure you tell them you are using CNC rears although with the bigger master it should not affect anything. You will have to adapt the larger master into the stock location unless Jamar or CNC has a direct bolt up, but I have always had to adapt them into my Bajas. They might recommend residual valves but I have never had to use them.

As a side note to this:
somebody probably already messed with the push rod length and that in itself can cause you have a pedal that feels like you have to pump it up, and the first push is half way down. The adjustment on that rod is a very sensitive adjustment and if not correct will cause the piston to not cover up the small orfices that are in the bore causing psi bleedoff. Sounds like only one half of the dual chamber is working anyway since you can not maintain some psi to the rear. Brake switch should be off the master. A Mighty Mite vacuum hand pump or similiar is your friend, go buy one the Harbor Freight cheap one even works great.
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[jeepsterryan]

oldschool5er - Thank you. That is exactly the info I was looking for. I didn't want to spend a bunch of money chasing down a problem when I wasn't even sure if I was chasing down the right problem.

As a note, I don't think the lever length has been modified/adjusted. Before adding the rear disc brakes, everything worked fine. The pedal throw was just right. Now it just doesn't engage until half way through no matter how many times you pump it. It just never builds the pressure in the rear chamber.

I'll get a larger bore cylinder coming today. For now, I'll get the vacuum pump and see if I can get it to work marginally well. We have been trying to get the emissions done. We finally have the jetting correct (we think), but we did the brake mod at the same time, so we haven't been able to drive it to the shop to see if we pass. It's been one thing after the other since we bought it and my son really just wants to drive the bug. We're getting close though.

I'm always grateful for everyone that takes the time to read and respond. It has been invaluable.

[mrbajadude]

I'll be running ghia front disc and Porsche 924 on the rear!! Will I need a different master cylinder or will the stock dual chamber work OK???? tia,John

[mrbajadude]

I'll be running ghia front disc and Porsche 924 on the rear!! Will I need a different master cylinder or will the stock dual chamber work OK???? tia,John

[oldschool5er]

[jeepsterryan]

Just wanted to give a quick update. We purchased a '71 and newer master cylinder and installed it. My research said that the master cylinder would work, but installing it was not a bolt-on ordeal. It required drilling new holes in the firewall. That section of the bug is double walled, so we had to add some spacers between the walls. Luckily, Lowes had some steel spacers in the hardware section that worked perfectly. Drilling the holes required the removal of the entire pedal assembly. We also had to shorten the plunger rod by about .5". And, finally, I had to purchase brake line Tee as well. The '71 and newer cylinder only has a front and rear port. The older cylinder had two ports on the front.

Once we finished we bench bled the new master cylinder, installed it, and bled the entire system. The brakes are better, be we still don't really build any pressure until about halfway through the pedal stroke. It's strange, but it stops now. so we're going to leave it at that.

Thanks for all the help.

[Bashr52]

[oldschool5er]

Keep in mind that Ghia Calipers as an example are single piston calipers so can get away with stock based master cylinders, but when you have dual pistons in one set of calipers and single pistons in another pair working off the same master, you will have issues especially if you are running different makes front and rear. You will have to get large bore masters and maybe even run residual valves on one side. Residual valves are not the best because they induce a minor drag, but might be neccesary when you are matching up bastard front and rear calipers. Even if single piston calipers front and rear some manufacturers use different size pistons which throw in another ? mark in the design. These are Brakes after all and your life depends on it being right. I am all for innovation and looking for alternatives, so with as many brake setups and combinations available in the market no telling what people can come up with that works..... but it is a slippery slope. Please be careful and look to manufacturers for design input IF they are willing to give it. I am sure they worry about liability.
_________________
HRE in Westerville,Ohio

Full machine shop with all types of CNC. Rapid prototyping available, CNC Plasma cutting, 3D printing and Laser engraving.