Master Cylinder & Brake Tech

Having an issue with our brake system....using CNC side by side master cylinder....here are the specs first..
M/W 4 piston calipers in the rear
Strange single piston calipers in the front
Funny Car Chassis
Hand brake lever ratio of 10:1

As recommended by CNC the 3/4 piston side plumbed to front brakes and 7/8" piston side plumbed to rear brakes (tried it reversed and the problem gets worse)

We are absolutely sure that all of the air is out of the system....removed the top caliper bolt and swung the top of the caliper until the bleeder screw is straight up to bleed...all fluid...no bubbles...

Here is the issue - the 7/8 piston side has so much stroke that the cross bar is in a bind....the 3/4 side stops immediately (very short stroke) and the 7/8 side cannot make correct pressure....and I do have the bais moved to the 7/8 (rear side)....both spacers toward the 3/4 rod end....we have a 2 lb. residual valve in the front and 4 lb. residual valve in the rear....both screwed in at the back of the master cylinder...

Currently making 600-700 psi on the rear and 400-500 on the front....and the car does stop just fine...we have been running it this way for 2 seasons....

Since we have plenty of time over the off season i was just wanting to correct the geometry issue and shoot for 1000 psi rear and 400-600 psi front...

I have talked with CNC several times and they suggest that there is still air in the rear system....we use their pressure bleeder with 5 psi to bleed since the M/C is below the calipers...

Is there something I'm missing? They suggested the 3/4-7/8 combo M/C and i get the same results with 2 different master cylinders...

The last alternative is to change to the Strange (Mopar style) master cylinder...but then I gotta weld on new brackets and make all new hard lines...kinda like the CNC master....



Any ideas appreciated....photos attached with brake handle pulled creating pressure....

 

i have the same deal on my car , but the bolt that joins the two rods together isnt on a angle like yours , it has spacers that keep it straight when you pull on the lever.

 

brake tech

Hi, first, 10 to 1 lever ratio is not enough for a hand brake. if you are now making 600-700 psi, thats about right fot that ratio. you will probably need around 15 to 1. also, do not use the mopar style strange master. There is a reason why the mopar style strange master is 100 dollars and their funny car master is over 500 dollars. The funny car master has a 2 piece rod inline with a spring internally to limit the front pressure no matter how hard you pull. that way you can alter the ratio to get the 1000 psi to the rear without changing the front press. been through this on 2 different funny cars. IMHO put the strange funny car car master on it, adjust the lever ratio to get 1000 psi to the rear and you will see the difference. Tom.

 

Too much travel

There is a good chance your rear caliper pistons are pulling back too far and requiring more fluid than the MC can deliver before developing pressure.
Install a 2 PSI residual pressure valve in the rear system and I'll bet it works a lot better.
I changed our calipers and pads to TBM brakes: http://www.tbmbrakes.com/racing
and all these problems went away!!
Steve

 

You have too move the rod from the hand brake too the center of the master cylinder with a spacer on either side right now you have both spacers on the right side so its putting all the pressure on the left side. Been there done that

 

The spacers are both on the 3/4 side so that the 7/8 side gets the most pressure - if I move the pushrod to the center (between spacers) then the rear gets even less psi...so both spacers on one side is on purpose to shift more pressure to rear brakes....

 

I just know no matter what you do if the rod isn't in the center its always going too push the one its closer too more an it will bind .Give your brake lever higher ratio an that will raise your pressure . Move it too the center an give it more leverage too raise your pressure.Sorry hope this helps

 

it sounds like this would make the front pressure higher than the op wants. You could make up an interface bracket. Shorten the brake handle rod a couple inches. Make a plate that spans the two brake rods and connects to the brake handle rod. Make a short solid rod between the plate and the rear brake rod. Make a telescoping rod between the plate and the front brake rod. The telescoping rod might want a spring so you don't get a bunch of rear brake before the fronts hit. Just a thought, hopefully the description makes sense.

 

cnc

I had a dual setup do that years ago. The side that was compressing more had a piston that was too small for the volume of the calipers. I think it made good pressure but acted like it had air in it.
As somebody else stated it might be the pads retracting excessively but I played with residual valves and other options before I gave up and bought a bigger bore master cylinder.

 

You may have a piston size/piston count mismatch front to rear (hardware mismatch). It takes different volumes to move 8 rear pistons than it does to move 2 front pistons based on size. The M/C piston/valve size offset helps, but may not be enough. What are the caliper piston sizes. Let me know and I can do some quick calcs as to how for the M/C has to move to actuate the front vs. rear.

 

Made a few adjustments last night and have better results....

The first mistake I made was not replacing the pressure bleeder top with the top that comes with the master cylinder....it needs to be vented in order to not damage the piston seal when brakes are applied....I got scolded by Corrina at CNC about this....

The issue was definitely the excessive stroke on the rear brake side (7/8") due to volume with 8 pistons so I screwed the plunger out a few turns (maybe 1/4") to let the cross bar be at an angle while at rest (handle forward)....now when applied the bar is almost straight across...giving the 7/8 side a "head start".....the 7/8 side plunger still goes in further than the 3/4 side....but they are aligned a lot better when the brakes are applied....by lengthening the pushrod the outer boot is not totally collapsed when the brakes are applied...

Also took the advice of Tom TC1216 and drilled a new hole lower on the brake handle....the Mckinney handle is about 25" from the bottom pivot to the center of the hand grip....old pushrod hole was 2.625" and now I have one at 2"....

Brake pressure on the rear is 800ish and 450ish on the front....but it's kinda hard to watch both gauges and pull the handle at the same time...will get Radar in the car next week and have him give it a good pull to report corrected numbers...but thanks for all of the replies...seems we are going in the right direction....