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Wilwood fronts and Neon rear discs... Which master cylinder???

Posted: Fri Mar 17, 2017 9:53 pm
by DanteGTZ
Finally got around to swapping my factory '93 brake master cylinder to a '93 Lumina Z34 master. I've got to say, I don't think it's going to work for my setup at all. For starters, my front brakes are the 95+ J-body Wilwoods and my rears are the 95-05 Neon rear disc setup. With the Z34 master, the pedal feels fairly stiff and high and the front brakes perform fairly well. Unfortunately, the rears are a totally different story. The rear brakes are extremely prone to locking in a panic stop or heavy braking scenario. On totally dry pavement (with near new tires), the rears locked almost every time I tried to do a hard stop. Not good. I thought for sure that if I were going to have any braking issues with the Z34 master, it'd be evident on the front brakes, not the rears.

If my car were a Pre-92 car w/o ABS I'd have an easy fix and could use an adjustable prop valve. With the ABS system, the master is obviously different. What else has everyone else used? Given my symptoms, can someone recommend a master that might work better?

Re: Wilwood fronts and Neon rear discs... Which master cylinder???

Posted: Sat Mar 18, 2017 11:52 am
by woody90gtz
If I remember correctly, the Wilwood calipers have less piston area than stock, which would throw off your bias. I know there is a thread where I did the math, let me find it.

Re: Wilwood fronts and Neon rear discs... Which master cylinder???

Posted: Sat Mar 18, 2017 11:53 am
by GT_Indy
I'm not sure if this would help but on the 90s Cadillac devilles and Eldorado I believe they had a flow limiter on each rear brake line to help delay the rear pressure surge when the brake was pressed fast with a lot of force.
I wonder if that would help.
Their positioning on the line as far as distance from each rear brake matters so they will be in two different places under the vehicle.

Re: Wilwood fronts and Neon rear discs... Which master cylinder???

Posted: Sat Mar 18, 2017 11:54 am
by woody90gtz
You did bed the pads before you did any testing, right?

Re: Wilwood fronts and Neon rear discs... Which master cylinder???

Posted: Sat Mar 18, 2017 3:11 pm
by DanteGTZ
woody90gtz wrote:You did bed the pads before you did any testing, right?


Yup.

On a side note, we did discover that the rear caliper pins were pretty dry so we lubed those up a bit more but I'd imagine that's just going to help them release and have nothing to do with the excess pressure.

Re: Wilwood fronts and Neon rear discs... Which master cylinder???

Posted: Sat Mar 18, 2017 9:18 pm
by woody90gtz
Ok, just added a post (viewtopic.php?f=11&t=24614) with tons of information.

I put your combo in the spreadsheet (assuming 93 Lumina ABS master puts out the same pressure as a 91 non-ABS) and it says you should be able to decelerate at 1.3G with 20,624inlb of brake torque, which is pretty good.
Brakemath Brian - stock master.xls
(28.5 KiB) Downloaded 26 times

It also shows that the weight transfer at 1.3G (if my 58/42 static bias is right) is 86/14% for brake torque when you have 80/20% available. So at max, I can see why it could lock up.

The problem is, your max rear brake torque drops to 2,673 with the stock master, which is only enough for .3-.4G. It requires 3,469 at 1.3G and 3,870 at 1G. So the fronts will have to band-aid the lack of rear stopping power, bringing your total stopping power down to appx 1.2G. Still not bad at the max, but you will eat front pads much faster in normal driving too because you have half the rear brake ALL the time.
Brakemath Brian - Lumina master.xls
(28.5 KiB) Downloaded 26 times

Here's a question though, and maybe it's a dumb one, but why does the rear brake lock at all if it is an ABS setup?

Re: Wilwood fronts and Neon rear discs... Which master cylinder???

Posted: Sat Mar 18, 2017 9:28 pm
by woody90gtz
According to the calcs mine should also lock the rear at max brake. I just happen to have a new set of tires waiting to go on, so I will do some testing and try and flat spot the old ones. haha