brake balance

[vw1303]

I have a 74 1303 with early 944 calipers all around. Under hard braking my rear brakes lock up quite easily. Even if I try i cannot lock the fronts up. Has anyone used a prportioning valve on the front brakes to cure this problem. I am using the stock master cylinder and have quite a stiff front suspension.

Thanks
Erik

[Michael Ghia]

Hi,
I've not had this problem but I used a Porsche master cylinder. Are you sure you have the front brakes bled fully?
I used a 911SC MC which is 20.64mm although I also used it with the booster from the same car. I found it impossible to lock up the rear brakes and they were very good.

There are those of us who believe that the stock MC isn't capable of supplying enough fluid to 944 calipers although this has been argued lots of times.

MG

[beanbooger]

I found it impossible to lock up the rear brakes and they were very good.

So this is a recommendation to use a 911 MC on a beetle (thing, actually) with 944 discs all around? Makes sense to me.

Thanks,

Brian

[MattKab]

Hi,

I suggest you try a split-bore (3/4" & 15/16") 944 master cylinder and perhaps that type proportioning valve on the REAR circuit.

This will push more fluid to the front calipers. The stock Bug m/c is only 3/4" bore front and rear.

Increasing the bore of the m/c will have no effect on the front/rear brake bias. It will only serve to increase the amount of fluid displaced for the same pedal stroke which will decrease the mechanical advantage of the system, which may not be such a bad thing in some cases.

Matt

[beanbooger]

Is the early 944 M/C the split-bore one that you are talking about?

Thanks,

Brian

[craazy Cooter]

Install the proportioning valve in the rear circuit. Easy fix.

[MattKab]

Yes the early one. Remember the larger bore that is rearmost of the m/c serves the front brakes.

Search out Supermans thread, for top info.

Matt

[Shad Laws]

Hello-

I agree that often times the split-bore 944 MC is a better match for 944 brakes than the VW ones... and the way you said to hook it up (big bore = front) is correct...

...BUT!!!!!!....

That will NOT solve your problems of the rears locking first. In fact, with all other variables equal, it will make them worse. Putting a bigger MC bore in the front than the rear makes the rear bias more, not less.

I suspect you have other problems in your system...

Take care,

[MattKab]

Shad's right, more fluid to the front will give a lower pressure, increasing the bias towards the rears.


Matt

[vw1303]

I am going to stick the stock master cylinder and buy a porportioning valve and install it in the rear circuit. Will post results on the weekend.

[Shad Laws]

Hello-

Your hydraulic leverage ratio is (54/36)^2 = 2.25. By contrast, stock Ghia fronts with the rotohub rears is (40/34)^2 = 1.38. Yet, you are locking up far more _frequently_ than those with rotohubs...

...this is not a good sign...

I'm fairly certain that there's something else wrong with your setup. A bias valve may bandaid the problem, but it's still there. With that HUGE of a change in bias ratio (from 1.38 to 2.25... that's 63% more!), your fronts should be locking up all the time.

Take care,

[Superman]

vw1303, you may want to do what the Porsche guys have done. The early 944 master cylinder doesn't have a proportioning valve but the later cars do. They take the late 33/5 proportioning valve and add it the early master cylinder. The part number is 928.355.305.02 and costs about $70 but it isn't adjustable like the Wilwood unit.

There are also a few other things to take into consideration. Make sure that you have the lines connected correctly because the 944 m/c is opposite then that of the 1303. This meaning that on the 944 m/c it has the front hole for the rear line. Have you checked this? If not, start there first.

There are also other options but try the above first. Porsche also used a 18/5 proportioning valve on the 944 but it was replaced with the 33/5 eventually. Also, some 944 owners have put a 23mm Mercedes Benz master cylinder on their cars but I have no details about that conversion.

Some other things to think about is if your rear tires are much larger than the front and/or if the car is lowered in the front. These can affect bias as well.

[vennard]

dont have any info to help you except, When it comes to porsche parts in a vw listen to shad. Been following a link on 901s and the guy nows his sh*t.

[vw1303]

I installed the wilwood biasing valve this weekend. It was raining so I didn't get a chance to fully test the new setup. I was able to lock the front brakes up when I decreased the rear line pressure by 25%. But I really need to test when it is dry. I have up to 57% adjustment on that particular valve. I think the reason for the strange bias was in my suspension setup. It is much stiffer than the stock suspension and my front end does not dive under hard braking like it did with stock springs.

[vw1303]

ok. it dried up today and i conducted some more tests. with the screw turned all the way out it was hard to lock up the rears but i lost alot of braking power. I can't figure out what the problem is. I have steel braided lines in the front and stock rubber ones in the rear. Could that be a factor? i guess the biasing valve was a band-aid, quite expensive one. Is there air in the lines? I have bled them numerous times to get rid of all the air. Thanks

Erik

[Ron Roberts]

If your front suspension is stiff and doesn't nose dive, then your front end is not taking on weight transfer. This means your front end stays light and should lock up easier than with a loose suspension. Your on the right track but I think you have it reversed. Air in the lines will result in a spongy feeling pedal. Iwould guess if anything braided lins in the front would help the fronts lock up. I think it was mentioned already, but make sure the front and rear is not reversed. In a rear engine car the front should lock up easy.

Ron

[MattKab]

I went for 6 months with Brembo's and stock flexi hoses, before trying Goodridge SS. I did the fronts one weekend, used the car for a week then I did the rears. No difference! I was surprised! But then again my Bug is no 951. If I hit the middle pedal, I hit the seat belt. My Maglite slides from the side of my seat frame/door and hits the innerwing and my anklebone. My ashtray closes...

Erik,

What spec. brake fluid are you trying? DOT5/silicone fluid can be a mare to bleed, I've heard. If your problem is not a bubble in the front pipes or m/c, it may just be the last thing you try!

A swelling flexihose is a good mechanical analogy of a capacitor. It can literally store energy, in the form of potential.

Maybe you have excessive residual pressure in the rear system, due to a weak brakehose? I saw something similar once on a stock T3.

Matt

[vw1303]

I am using DOT3 fluid. There may be one other thing. On the master cylinder I attached the front circuit to the forward most connection on the master and rear circuit to the furthest back connection. If I had these reversed that could be my problem. If I have connected these backwards please let me know.

Thank You

Erik

[Ron Roberts]

Sounds like you have it hooked up bacwards. Read supermans post on this thread. His waws the 11th reply.

Ron