# MK1 rear disc conversion



## fast8vvw (Mar 23, 2002)

I want to change my rear drums to disc. can any one tell me what i need to do this and where i can get some of the parts. or if you have the parts and are close or dont mind shipping them i might take them


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## Racer_X (Jul 14, 2002)

*Re: MK1 rear disc conversion (fast8vvw)*

This is not an easy modification, and it probably has more negatives for most folks than positives.
First, unless you are running upgraded rear shocks and heavier rate rear springs, I wouldn't bother. The extra unsprung weight (about 6 1/2 pounds per side) is something you will feel, and the handling of your car will suffer some from that.
The improvement in braking is minimal. If you aren't experiencing extremely high rear drum temperatures and serious fade from your rear brakes, you probably won't notice the difference. Do your drums smoke after a "spirited drive" or track session? If not, the benefit is going to be mainly looks.
The upgrade might be worth the trouble if you are:
- Drag racing (and serious about reducing rotating inertia)
- Regularly driving over 100mph and applying your brakes very hard more than once every minute (on track road racing)
- Someone who just has to have the really "cool looks" of rear discs.
To do it you need:
- Spindles from any A2 with rear discs or 16V Scirocco (A3 and A4 spindles might work, but I've never used them before)
- Rotors (and bearings) from a 16V Scirocco or an A2 (or later)
- Calipers and brackets from a 16V Scirocco or an A2 with rear discs (again I've heard later calipers work, but I haven't tested this theory)
- Parking brake cables for a 16V Scirocco (those are the only ones that work in an A1)
- Hard lines on the rear trailing arms from a 16V Scirocco (or custom make your own)
You also need to do something about the proportioning valve. 
You must remove the A1 "combination valve" on the firewall or fenderwell below the master cylinder. In it's place, you have two options. Both require some custome brake line fabrication, and there are tradeoffs here.
Option 1 - Proportioning valves from a 16V Scirocco. These screw into the master cylinder and the brake lines for the rear brakes attach to them. This option is easier to install, but these prop valves are very rare in the used parts yards and somewhat expensive from dealers and parts suppliers. Also, they are fixed pressure and slope proportioning valves, so if you carry 4 passengers or heavy loads in your Rabbit, your weight balance might not match the brake bias and you might have rear lockup problems (with light loads) or longer stopping distances and excessive wear and heat at the front (with heavier loads). No matter what, you'll have to fabricate the hard lines or get hard lines pre-bent for a 16V Scirocco to use these prop valves. Also, if you ever do rear discs on an A1 Jetta (or pickup for that matter), this method is very sub-optimal because of the weigh balance differences between the Scirocco and the Jetta (or pickup).
Option 2 - Use the suspension mounted, "load sensing brake pressure regulator" from an A2. This requires fabrication of some bracketry to hold the regulator in place, and also an actuator arm for the spring to attach to the rear axle beam. The fabrication work isn't that hard, but it's not as easy as just screwing in Scirocco prop valves. These regulators are much more plentiful in the parts yards, too, so the parts are cheaper. You've then got to make custom hard lines for everywhere. Also, you have to adjust the thing once it's installed. If you do this right, the result is better than the Scirocco valves, but it's more work.
If you're really set on doing this, bolt a 5 pound weight to the lower shock bolt on each rear shock and drive your favorite "spirited driving" roads (freeway entrance ramps or exit ramps or whatever). If the weight doesn't bother you and the work doesn't scare you, go for it. 
BTW, I have rear discs on my diesel Jetta (with the A2 load sensing pressure regulator), but I've also cut and drilled about 1 1/2 pounds of cast iron out of the rotor (mostly from the top hat and wheel flange area) and almost another pound from the other parts. I'm also running the lightest 13x5 aluminum wheels I could find in the junkyard, and I'm seriously looking for aluminum calipers now. And, I have springs that are slightly stiffer than GTI springs on it with reasonably good shocks. 
Without the lighter wheels and the other weight reductions, the handling with the rear discs sucked. I do get slightly better accelleration now due to the reduced inertia, and with the lighter parts and lighter wheels, the handling is acceptable again.


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