Cant get a good brake peddle (rear brake problem?)

[Bobalos]

Morning folks, my PSD has a dual MC setup with swinging peddles & a balance bar.  I cant get a good peddle on it @ all.  I thought it was simply a function of bleeding them out, but Ive bled them a bunch & while I can get it "better" its not by any means, "right".  Im fairly certain that is the rear by looking @ the balance bar & seeing how far the piston goes into the MC. 

Here is how its setup.  Is it right that the rears with the larger Calipers should have the smaller MC bore? 

Rear MC: 3/4", 6 piston Gear one calipers

Front MC: 7/8", 4 piston Wilwood calipers, (remote reservoir)

Throwout MC: 3/4"

Im Fairly certain that I have all of the air out.  I have bled it so much Im getting brand new clear fluid out of the bleeders @ the calipers.  I used my vacuum pump to suck as much air out as possible & then the wife & I did the "pump & hold" on it quite a few times.  She's done this with me for almost 30 years, she knows what a good peddle should feel like & she gives me pretty good feedback. 

FWIW, the rear calipers just came back from @parker@gearone who put bleeders in the tops of one of the calipers that did not have them & rebuilt both of them, so Im pretty certain that the calipers are all good.

Im have convinced myself that the MC for the rear is just crap.  if its air you can normally pump it a few times & the peddle feel, gets better, but this never does.  its crap every time you push it. 

Does anyone know if anyone sells rebuild kits for 3/4" CNC master cyls?

Does anyone know if there is an off the shelf Master Cyl that has the same bolt flanges as the CNC products, so I can replace these?

Does anyone have any suggestions, on what to try next to see if I cant figure out if the MC truly is dead?  if I put a gauge on it, what kind of pressure should I bet able to get out of the MC? 

Below are some pictures of what it looks like if that helps any....

thanks

Bob





 


 




 


 




 


 

 

[parker@gearone]

3/4" master cylinder is to small for those 6 piston calipers, 7/8" might be okay, but I would replace it with a 1" MC. Call Denny at foddrills he should be able to help you find a direct replacement.

 

[Agricon]

I looked everywhere trying to find rebuild kits for these, no luck. So I just measured and bought the cups and boots at napa and on line. I rebuilt the 3 masters and turning brakes plus the clutch slave for like 25 bucks. Parts are cheap if you don't get the kits.

 

[danbalz]

CHECK FRONT AND REAR WHEEL BEARINGS/HUBS FOR SLOP. FRIEND HAD CRACKED SPINDLE THAT CAUSED BAD BRAKE PEDAL.

 

[Fullthrottleguy]

As Parker said 3/4" is too small  with 6 piston - you just cannot get enough volume through -  you either have to have a Larger Diameter Piton in the MC or have a longer travel on the Pedal to get the movement on 6 pistons.  In you case the Pedal feel is bad because you have to move the pedal so far.  Also add that the likely 4 piston front brakes could do OK with a 3/4 but with a 7/8 the pedal throw in small.  

I like the 6 piston GO set up it it has an excellent pedal feel and incredible stopping power.

 The simple fix for you  is a 5 minute job ---Swap the lines and put the 3/4 on the front and 7/8 on the rear and that shoulf get you what you want - since a Sandcar (assuming you don't do alot of desert driving) is biased rear to front unlike a street car, and uses the rear brakes more than the front  It will be fine and in facts its how I set up most cars.  If you are not good at setting the balance bar, you can set with it centered and adjust after short test runs. 

a 1" MC will shorten the throw even more, but the rule is the bigger the Diameter of the MC piston the "harder" the brake pedal is to push.    a bit size different make a lot of pedal difference. Most swing pdeals have a better ratio (long lever arm) than floor mounts and a 1" is probably OK, but a 7/8"  1"  combo is going to be hard to push although the travel in the pedal will be likely less than 1" inch

One more thing - it looks like to have minimal "soft lines"  ands thats good - Even Braided soft lines will make the pedal spongier if more than 24" long  ...

 

[Turbo_Manx_Maniac]

Do you have a residual valve in there for the rear brakes? 

Do you have cutting brakes?

 

[Bobalos]

Thanks folks.  

Thwre are no cutting brakes and i dont think there are any residual valves.  If there are they are hidden under the center console with all of the tubing and Crap in there.  

I will call Denny in the AM and see if he can help me out. 

 

[Fullthrottleguy]

Thanks folks.  

Thwre are no cutting brakes and i dont think there are any residual valves.  If there are they are hidden under the center console with all of the tubing and Crap in there.  

I will call Denny in the AM and see if he can help me out. 

You should not need a residual valve with swing pedals - the CNC MCs have a Check valve. The residual is only needed when the Brake MC is lower or equal height to the wheel cylinders  - like if you have floor mounted pedals  - the swing pedals are almost always above the wheel cylinders.  Some people use them, becasuse they have always used them when they had floor mounted pedals and its a habit 

You should always bleed the cutting brakes, but the simple way to do this not to use the bleeder screw on the cutting brakes - that bleeds air coming in - but you can do the same thing faster by applying the cutting brake while you are bleed the rear brakes (Hold it open) that makes the cutting brake a pass through = just do that on both sides and you won't have to open the console.  I do that in every car with no issues (unless the cutting brake is bad and bleeds down of course) - 

 

[L.R.S.]

Jamar master cylinders are direct bolt on replacements for CNC.  Changed my clutch master a few years ago.  It's a little taller is the only difference.



My sales guy Jeff Black at Kartek told me CNC was closing and he offered me what he had in stock for rebuild kits.  I said I was good and didn't buy anything.  Bad call because the very next year one of my brake masters went bad.  Called around and no one had the kits.  Heard from someone (can't remember who) that Butch's Speed Shop in Vegas had some so I called and sure as sh!t they had a lot.  I bought a rebuild kit plus two extras.  In hindsight I should have bought everything they had in stock.

 

[lincster]

Jamar will work in your application.

With my hanging pedals, the way my builder installed everything, the Jamar would not bolt directly in place of the CNC.  The reservoir would hit a bar, so I had to redrill the holes.  But with how your cylinders are set up, it should be an easy replacement.

I bought some spare rebuild kits from a local shop that still had them.

I can check the size and let you know if you are interested.

I switched all of my stuff to Jamar so don't need the rebuild kits anymore.