# MK2 REAR DISC CONVERSION PROBLEM



## _Dubaholic (Oct 28, 2003)

I installed rear discs on my 92 gti 8v and when i brake really hard my rear wheels lock up and the back end starts to fish tail. can someone please help me and tell me why this is happening.


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## HilF (Jan 10, 2004)

*Re: MK2 REAR DISC CONVERSION PROBLEM (_Dubaholic)*

your proportioning valve is
a. a drum setup
b. not adjusted correctly. the rears are getting more braking bias now and it is causing them to lock up.
what you want to do is:
a. get a rear disc prop. valve
b. adjust it to that you're simulating a lighter load in the back - the rear isn't as low. when you jack up the car, the rear beam 'falls'. you need to adjust the prop. value into thinking you're higher.
have you lowered your car?
should just be 2x13mm bolts to adjust the prop. valve. http://****************.com/smile/emthup.gif


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## _Dubaholic (Oct 28, 2003)

*Re: MK2 REAR DISC CONVERSION PROBLEM (white_r!ce)*

well i bought a new prop valve from a very well respected vw performance shop and they said that both prop valves were the same for both drum and disc a2's. i have also asked other sources and the answer is the same.yes i have lowered my car. to correct this would i adjust the bolt upwards or downwards on the adjustment bracket? i had it adjusted half way before and i didn't have this problem but i felt like the rear was doing more braking then the front IE my rear rims always had more brake dust then the fronts. i now adjusted it 1/4 from the bottom of the braket. and it's a 13mm bolt with a 13mm nut on the other end. would i have to change my master cylinder if i was going to do this conversion? someone please reply


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## Racer_X (Jul 14, 2002)

*Re: MK2 REAR DISC CONVERSION PROBLEM (_Dubaholic)*

Do you have the load sensing brake pressure regulator on the rear suspension? Or does yours have the inline proportioning valves at the master cylinder?
With the load sensing brake pressure regulator, you can adjust the bias on the brakes. The harder the pin from the suspension pulls on the spring, the more rear bias you will have. The less the pin pulls on that spring, the more front bias (less rear brakes) you will have. Release some of the tension on the spring if your rears are locking up. This can be a big problem if you lower the car. The regulator thinks the lower suspension is due to a lot of weight in the back, so it cranks up the rear brakes. But since it's not really weight causing the lower ride height, the rear wheels lock up from the extra brake pressure.
Also, make sure you don't have any inline "residual pressure valves" anywhere in the rear brake lines. Those are commonly found below the master cylinder along the firewall in the rear lines. 
And the regulator block is different on disc brake equipped cars. The drum brake regulator is cast iron and is part number 191 612 151. The disc brake regulator is a machined aluminum part and is part number 191 612 151C (I think, though it might be 151A). They are not the same and using a drum brake regulator with rear disc brakes will cause problems.


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## _Dubaholic (Oct 28, 2003)

*Re: MK2 REAR DISC CONVERSION PROBLEM (Racer_X)*

I'm 99% sure that the inline pressure regulator is aluminum. that is the one that i have installed. i don't have a proportioning valve at the master. can the spring ever be adjust so much that there will be more braking power at the rear wheels than the front? I ask this because i would get more brake dust on the rear rims then the fronts but i'm thinking maybe it has something to do with the type of brakes i have on there as well. someone please reply


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## _Dubaholic (Oct 28, 2003)

*Re: MK2 REAR DISC CONVERSION PROBLEM (Racer_X)*

just to confirm i have the load sensing brake pressure regulator on the rear suspension.


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## HilF (Jan 10, 2004)

*Re: MK2 REAR DISC CONVERSION PROBLEM (_Dubaholic)*

maybe the pads are different in the rear and have a higher friction coefficiant (err..got too scientific for myself there)
basically, pads in the back might be better than the ones in the front?


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## _Dubaholic (Oct 28, 2003)

*Re: MK2 REAR DISC CONVERSION PROBLEM (white_r!ce)*

even so there still shouldn't be wheel lock up in the rear. The pads can only make so muchof a difference because there should still be much more brake pressure on the front


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## Racer_X (Jul 14, 2002)

*Re: MK2 REAR DISC CONVERSION PROBLEM (_Dubaholic)*

OK, here's some information on the "load sensing brake pressure regulator."
The load sensing *regulator* is a pressure limiting device. Up to a certain cutoff pressure, 1 PSI of brake fluid pressure at the inlet ports gives 1 PSI of brake fluid pressuer at the outlet ports. Above the cutoff pressure, no additional pressure goes to the outlet ports. It's kind of like a proportioning valve with a slope of zero above the cutover point. The cutoff pressure is the maximum amount of pressure that goes to the rear calipers. If you press the pedal harder beyond that cutoff point, all additional pressure goes only to the front brakes. 
The tension of the spring on the lever sets the cutoff pressure. More tension on the sprnig=higher cutoff pressure. Less tension on the spring=lower cutoff pressure. 
In order for it to work properly, the lever on the regulator block must be free to move. If it's seized up internally and the lever won't move, the regulator won't work. If you have the lever bound up on something externally and it can't move, the regulator won't work. 
And, yes, if you put too much tension on the spring, you can get to the point where the rear brakes will lock up before the fronts. If you adjust the regulator spring too tight, or you lower the car without releasing the additional tension that put on the regulator spring, the rear wheels will lock up. If you have it adjusted really close to the limit for your rear wheels (for optimum braking) and you upgrade the rear pads to Ferodo DS2500 and keep the Mintex Red Box pads on the front axle, you might also get rear wheel lockup because the total braking force at the rear wheels increases by 50% or so with that pad upgrade, and the front doesn't get a similar increase in grip. So for the same pressure, the rear brakes will do a lot more braking in that situation. Either put similar pads on the front or release some tension from the spring (to lower the cutoff pressure) if you do something like that with the pads.


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## Vdubs (May 16, 2001)

*Re: MK2 REAR DISC CONVERSION PROBLEM (_Dubaholic)*

Adjust the regulator or get a set of gauges to set up the pressure, usually on ebay cheap ish.
Btw, the alloy regulator is a direct swap (supersession) to the steel version, it is supplied for the drum brake and disc brake from VW, the steel version is no longer available, early cars with rear discs did come with a steel regulator they were not all alloy, just like the 89 16v Golf in my shop.
Etka lists the 191 612 151c as deleted 2/89 and to use the 191 612 151.
Mk3 has 2, 1HO 612 151 drums, 1HO 612 151C disc.


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## _Dubaholic (Oct 28, 2003)

*Re: MK2 REAR DISC CONVERSION PROBLEM (Vdubs)*

thanks a lot guys for your input. I'm going to adjust it one time and i also noticed that the ebrake cable is "wrongly" routed and sittinbg to close to the pressure regultor sprong and it looks like it might be getting caught up on it. I'm going to reroute it and see if that makes a difference because if the cable is intreferring with the spring then when i brake and the spring wants to move it might not be able to work properly with the cable in the way. I will let everyone know how it works out.


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## Racer_X (Jul 14, 2002)

*Re: MK2 REAR DISC CONVERSION PROBLEM (Vdubs)*


_Quote, originally posted by *Vdubs* »_Adjust the regulator or get a set of gauges to set up the pressure, usually on ebay cheap ish.
I won't argue that the official way to adjust it is with brake pressure guages, provided you have a stock suspension setup and the entire stock brake system with stock pads all around. 
I've had better luck setting the regulator by trial and error, first on an empty gravel parking lot, then on an empty paved parking lot. If you customize the actuator arm on the suspension (shorten it or lengthen it), you can even adjust the gain for weight and get it more properly tuned to your vehicle. That's very important if you replace the rear springs with springs that are a different rate than the stock springs (stiffer or softer rear springs). 
The field test method is also useful if you're putting rear discs on a car that was never offered with rear discs (like an A1 Jetta or Rabbit) and therefore there is no factory spec for the pressures when setting them with pressure guages.

_Quote, originally posted by *Vdubs* »_Btw, the alloy regulator is a direct swap (supersession) to the steel version, it is supplied for the drum brake and disc brake from VW, the steel version is no longer available, early cars with rear discs did come with a steel regulator they were not all alloy, just like the 89 16v Golf in my shop.
Etka lists the 191 612 151c as deleted 2/89 and to use the 191 612 151.
I suspect that more information from a technical note somewhere is necessary. The older cast iron one defintely had a residual pressure section in it, with springs and valving that isn't present in the aluminum one. That valving is required for rear drum brakes to function properly. Also, that valving would cause rear disc calipers to drag if you use that cast iron regulator with rear discs. 
I suspect there's a tech note somewhere that specifies you can use the aluminum block *plus some inline residual pressure valves* in place of the older cast iron regulator block/residual pressure valves combination. It's probably not a direct swap. If you have access to ETKA and/or the tech notes (on microfiche), I think you'll find that it's a little more complex than a direct supercession.

_Quote, originally posted by *Vdubs* »_Mk3 has 2, 1HO 612 151 drums, 1HO 612 151C disc.
Correct. The 151 unit includes some internal residual pressure valves, the 151C unit does not have the internal residual pressure valves.


_Modified by Racer_X at 8:23 AM 11-28-2004_


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## Vdubs (May 16, 2001)

*Re: MK2 REAR DISC CONVERSION PROBLEM (Racer_X)*

89 Golf 16v disc, original.
















90 diesel Jetta drum, replace with alloy unit.








We've been fitting the alloy replacement for years on drums, they all work exactly the same, never had any problems and I see the same cars year after year.
The MK3 is an entirely different part to the MK2.



_Modified by Vdubs at 1:33 AM 11-29-2004_


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## Broke (Aug 13, 2001)

*Re: MK2 REAR DISC CONVERSION PROBLEM (Vdubs)*

I looked around in ETKA at the prop valves, and it seems all Jetta cars got the beam mounted, and the base Golf cars got the inline versions near the MC, with the GTI Golf having the beam mounted like the Jetta.
There is a change noted in ETKA in 1989 (and is continued to be noted up through 1992), when the base Golf car started using the same prop valve as the other models. The break is shown as F 1G-K-005 316 <<>> F 1G-K-005 317, with cars up to number 316 using a smaller drum (180x30mm) and cars numbered 317+ getting large drums (200x40mm).
The drum size change is noted in the mexican Golf listing, and not in the USA Golf listing....both overlap in the 1989 year. For 1989 German Jetta, they came with both the small and large drums, with the change being for the RV engine models from car #280+, which got the larger drums. Either German Jetta got the beam mounted valve. The 1989 Jetta rear disc seems to be ABS only, but the 1986 german Jetta rear disc car uses the same beam mounted valve as the drum car (180x30mm, the larger wasn't available in 1986).
The beam mounted valve was shown as 191.612.151 and 357.612.151, and it seems the change was the mexican cars used the 357 part, and the US and german cars used the 191 part. The old number was 179.612.151.....the 179 changed to a 191 in 1985, with the MK2. The 191 then changed to a 357 for the 1989 cars from mexico.
It seems the check valves that go inline that Racer-X is talking about was used on the Mk1 cars, like the 79-84 German Jetta, Rab Conv, Scirocco, ONLY with the use of the 176.612.151 beam mounted prop valves. 
Once the 176 number was changed to the 191.612.151 parts (1984), the check valves (113.611.191) were no longer needed. Since 1984, the 176 prop valve hasn't been used, the 191 and 357 valves have been used, and those are used on drum and disc cars both, and do not require the use of the check valves.
Whether the older 176 beam mounted valve was used on drum, disc, whatever during the MK1 years, I don't know...I know in 1984 it was gone, replaced with a 191 valve, and the check valves died with the 176 beam mounted valve.....cars then either got the 191 beam valve, or if it was a base Golf, it got load sensitive inline prop valves, one in each line, right at the MC. Later those died with the onset of the larger drums.
Basically, if you buy a beam mounted prop valve made after 1985, it'll work on both the drum and disc cars. If you find an old 176 beam mounted valve, it needs the check valves.
For the MK3 prop valves, they have one for the beam cars, one for the drum cars, but for MK2, both are the same. http://****************.com/smile/emthup.gif
Broke


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## _Dubaholic (Oct 28, 2003)

*Re: MK2 REAR DISC CONVERSION PROBLEM (Broke)*

well i have a 92 gti and i was told that the prop valve was the same for both drum and disc cars of course i assumed there shouldn't have been a problem getting a new prop valve and worrying about if it was the right one or not. I think my problem is adjustemnt and because my car is lowered. thanks a lot for all the inout guys. I'll let everyone know once i figure it out


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## Audi4u (Jun 29, 2002)

*Re: MK2 REAR DISC CONVERSION PROBLEM (_Dubaholic)*

could just be bad calipers. they are know for that. I had a caliper go bad on me once just because i took it off to paint it.
The calipers you put on could have dirt and water in it from sitting.


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## _Dubaholic (Oct 28, 2003)

*Re: MK2 REAR DISC CONVERSION PROBLEM (Audi4u)*

yeah but the rear calipers were just both rebuilt before they were put on.


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## Audi4u (Jun 29, 2002)

*Re: MK2 REAR DISC CONVERSION PROBLEM (_Dubaholic)*

Maybe they messed up. On eway to check is to rotate the wheel with the emergency brake off. if there is tension on it then there are sticking.
If not then is the proportional valve.


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