I have a 1988 J65 (four wheel disk) car and I just converted my front brakes to a 1LE setup but I left my J65 master cylinder and proportioning valve in place. I know that it is recommended that I change to the J50 (front disk rear drum) master cylinder and porportioning valve but I want to know what the difference is? With the stock J65 parts the car stops well but I have to use high pedal pressure to stop the car hard. The wheels will lock but I am pushing on the pedal almost as hard as I can to get the wheels to lock. I am making an assumption that the J50 master cylinder has a smaller piston so I can generate more line pressue with less pedal pressure? Does anyone know what the piston diameter of the J65 and J50 master cylinder is?

Thanks

 

The pre'89 J-65 (4 wheel disc brakes) master cylinder has a piston diameter of 25.4mm (1.00"). The J-50 (and J-65 beginning in 1989) master cylinder has a piston diameter of 24.00mm ( .945").

The high pedal effort you have described is consistent with what I experienced when I first did my conversion.

This subject has been covered in great detail in my CD.

Thanx,ANDYZ28

 

Hi All. I have scoured the boards for more information on the J65/J50 brake pedal feel issue. I understand that I need the 15/16" 24mm bore master cylinder for my brakes (I have upgraded to SSBC 1LE system and the brakes s**k). The issue im having is that I can NOT find a J50 master cylinder ANYWHERE. I bought 3 so far, all promising 15/16" bore only to find that they are in fact 1.25"! Can someone please help! Does anyone know where you can get a J50 MC? my most recent purchase was a Wagner MC108141 http://www.rockauto.co.uk/catalog/ra...ktemplate=true


I cant match any for sale parts to this list http://www.3rdgenformula.com/modification/brakes.htm


I am completely out of options and do not want to spend any more money on buying master cylinders to solve my braking problem.

 

The 1.25" bore area is the quick take-up feature, not the bore actually used once the brakes are applied.

RBob.

 

Hi Rob,

So the measurement of the bore from the rear where it connects to the booster is 1.25". Is that not the bore diameter? Guys on other boards talking about the j50 say the metal lip inside on the 15/16" bore is bigger than those on the larger bore j65. Not seeing that on this part I got from Rockauto.

 

The M/C has a larger outer diameter where it bolts to the booster, that is because it is a quick take-up design. When the pedal is first applied this larger bore quickly pushes the low drag caliper piston design for the pad against the rotor.

Once the pad contacts the rotor the smaller bore is then used to create the higher pressure to apply the brakes.

RBob.

 

Wow, so this MC in fact could be the desired J50 15/16" bore? Do all of these MCs have the quick take up design, and is there any way for me to confirm that this is in fact a 15/16" bore without taking it apart? I thought i could just measure the back where it bolts up but now it seems that was completely wrong. I really want to be sure I have the right prt before I go through bolting it up and re bleeding the system and I know there are a a bunch of guys here that would love to know how to confirm a j50.

 

I don't know of a way to measure the main bore's diameter without taking it apart.

RBob.

 

So I disassembled the unit by removing the retaining clip and here is what I found. You are correct about the step bore (I was not aware of this) the outer bore or secondary bore is at 1 1/8" where it meets the power booster and then it changes to an inner bore that looks to be 15/16" does this mean that it is a J50?


I am trying to improve my braking as I have the infamous "hard Pedal" feel issue, need to practically stand on my brakes to lock them up, and I am trying to find the J50 master cylinder with the smaller 15/16 bore and decreased pedal pressure (which increases pedal throw).


Does anyone know if these are the outer and inner step bore measurements for the "correct" master cylinder to solve this brake feel problem?


Please Help!

 

I'm not sure if that makes it a J50 M/C or not. However, an '89 is supposed to be the 15/16" M/C. Whether it really is or not I can't really say.

I do know that the 15/16" bore (24 mm), is the smaller of the two that GM used. This will provide higher line pressure at the same pedal pressure. By about 12% more.

Can swap the one you have on and see if it makes a difference. With the right tricks this can be done without having to bleed the entire system.

The other side of this coin is the SSBC calipers and pads. I have not been able to find, even on SSBC's web site, the piston diameters of those calipers. This is critical to finding where the issue is.

First thing Monday get on the phone to them and ask what the piston diameter is. Post it here and we'll calculate the area of them.

Also, what pads are you using?

RBob.

 

Hi RBob, I did as you said and here is what I learned:


Kit Used SSBC 82-92 FBody
Super Duty Brake Kit A123-2
Caliper: Dual Piston, 38mm
Rotor: 11.85" x 1" Vented
Install Guide Here: http://ssbrakes.com/files/2015%20Ins...ons/a123-2.pdf
Summary of the Kit here: http://ssbrakes.com/i-10092298-disc-...or-a123-2.html


As for Pads, these were the ones I just picked up http://www.autozone.com/brakes-and-t...48_16108_1845/
Prior to these I had semi metallic same style pads, if I am remembering correctly.


I took your advice and kept the Master Cylinder I bought
Wagner MC108141 with what should be the 15/16" (24mm) Bore per what I was told.

Amazon.com: Wagner MC108141 Premium Master Cylinder Assembly,: Automotive

Amazon.com: Wagner MC108141 Premium Master Cylinder Assembly,: Automotive

However, the part that came in the box does not have the E-Z bleeder which initially raised my concerns.
Not sure if that is specific to the J50 or late Model J65 with the 24mm bore.


Could really use your help figuring this out. I have read many boards where other members ask about this exact "hard pedal" problem with no resolve, so i'm hoping this thread will help both myself and others that are dealing with this same issue. Thanks!

 

OK, those caliper pistons are small, that is the main issue with the hard pedal. Secondary is the pads, from my experience ceramic pads don't have much compressibility. And they usually aren't rated high enough to be a good street performance pad.

By going with the SSBC front calipers with the slightly larger diameter rotor, you lost 16.5% of the front brake torque over the stock brakes (when using the 15/16" M/C). This is using the same pad coefficient of friction between the two setups.

I'm not sure of the actual friction rating of the AutoZone Max brakes pads. They just show a quick chart that rates them higher then the other ceramic brake pads they sell. So going with a pad such as Stoptech High Performance Street pad may or may not help.

I have those on two cars and they are on the high side of the FF rating.

As for the M/C you have (MC108141), if the current one on your car has the same bore, it won't make a difference. However, if the one on the car has a 1" bore, you will gain about 10.7% of brake torque.

Going to a 7/8" bore M/C and the brake torque is the same as the stock setup (with 15/16" M/C and stock caliper/rotors).

RBob.