View Full Version : Why are my rear calipers seeing a huge increase in psi over the fronts?
Wilwood w6a front and w4 rear with wilwood 7/8 bore aluminum master, pedal ratio is 6:1
manual brakes.
with a gauge my fronts read about 700 the rears see 900 to 1100 this is with the prop valve full open
i just dont understand why the rear brake circuit is giving more psi with longer run lines and a smaller fluid reservoir in the master.
car stops ok but not great and i cant lock up my brakes. No air in the system and the pads are def bedded in. No clue what to do as i about to switch things to a power set up.
my legs are fairly strong but the braking is def not confidence inspiring considering how fast the darn car is.
had my installer call Wilwood many times over this and there isnt a real explanation they offered. Im wondering if they sent the wrong master cylinder or something. How can we check? Is there a part number stamped somewhere?
Bryce
04-05-2014, 01:44 PM
Some MCs have a built in bias. Like a 60/40 split. Any chance the lines are plumbed wrong?
carbuff
04-05-2014, 01:55 PM
^ That would be my guess...
Def not plumbed wrong im using an Art Morrison kit which came with a bracket and cnc lines for the master and prop valve it can only install one way. Had a few friends over earlier who drove the car and said it feels fine i guess im really use to my touchy power brakes on my modern sports cars.
David Pozzi
04-06-2014, 10:35 AM
If it's their Chrysler aluminum master cyl with two screw caps. It might have step bore. Is the front port plumbed to the front calipers?
Like this? http://www.performanceonline.com/MOPAR-DISC-CONVERSION-MASTER-CYLINDER-MC39178/
silver69camaro
04-07-2014, 05:33 AM
The Wilwood MC can come in a couple types, but the more I think about it, neither should be a step bore. This is puzzling.
I think I would put a pressure gauge at the MC outlet, both sides, and compare those readings to what you have at the caliper.
Guys here are two photos. Matt from Art Morrison i am using your bracket and prebent lines which came with a wilwood prop valve. Part # 26011190 in your catalog. I attached a photo of the kit and of my car from months back during assembly
This is beyond crazy if you ask me. My Master cylinder is Mustang Manual brake specific in a 7/8 bore they don't offer it in any other bore. My pedal is the correct ratio as well.
dontlifttoshift
04-07-2014, 02:22 PM
Full open is with the valve turned all the way in, clockwise.
It seems the only way you could get that kind of pressure differential is if the valve is cutting pressure at some level and it is entirely possible that the lines are plumbed backwards _after_ the valve.
Guys the lines are not backwards i just checked for the tenth time you cant mix this up because everything is labeled. Look at the photo under my hood the lines on the left of the master toward the back are each front caliper marked FO and FO and the line in front of that is the rear marked RO. On the right of the master those lines are cnc bent and can only go one way no exception. FI and RI
The prop valve is supposed to cut down on pressure to the rears only and i have it turned about 1 full turn out from all the way in. Can not get front brakes to lock, rears lock no problem
My only assumption is my master or valve is defective i honestly have no other idea why there is such a deviation
dontlifttoshift
04-08-2014, 03:44 AM
Then, like Matt said, check pressure right at the master cylinder, .......weird problem.
Are you using the same gauge, or a different gauge front and rear?
silver69camaro
04-08-2014, 08:32 AM
You're right, the lines would be very difficult to install backwards - but even if you did, the pressure output ratio is 50/50. I'm not one to start throwing parts at a problem without knowing the root cause, but the only thing I can think of is to try another MC. I would still check the pressure at the MC however.
How is the end result? Can you get the brakes to bias correctly? One time I built a transmission where the pressure readings were all low. I pulled my hair out trying to figure out why; multiple gauges, valve body in and out 50 times, rebuilt again - never did figure it out. But it worked great and never had a drivability problem. What I'm saying is the end result may tell you what to do.
well i drove the car to another friends shop this morning and just stopped by and i think they found the loose end to this puzzle and boy is this crazy. it appears someone took a 69 power brake pedal for an automatic trans car and cut the bottom off and welded a manual brake pedal section on possibly from another year or even a Cougar. therefore all along we are thinking i have the correct manual pedal in my car but its a power brake pedal that was modified to work with the clutch pedal assembly. therefore my pedal ratio is way off and this has to be why im not getting the fronts to work right. the shop located a correct 69 pedal manual pedal and i should have that installed this Saturday. Il report back but after pulling my hair out i think this is the root cause of the issue. Unreal the stuff you uncover on these old cars it never fails to amaze me what someone was thinking way back when they did this. the shop that i brought it too today just happened to have a 4 speed manual brake 69 Mach 1 original car in their shop and this is how the discovery was made.
andrewb70
04-08-2014, 11:02 AM
well i drove the car to another friends shop this morning and just stopped by and i think they found the loose end to this puzzle and boy is this crazy. it appears someone took a 69 power brake pedal for an automatic trans car and cut the bottom off and welded a manual brake pedal section on possibly from another year or even a Cougar. therefore all along we are thinking i have the correct manual pedal in my car but its a power brake pedal that was modified to work with the clutch pedal assembly. therefore my pedal ratio is way off and this has to be why im not getting the fronts to work right. the shop located a correct 69 pedal manual pedal and i should have that installed this Saturday. Il report back but after pulling my hair out i think this is the root cause of the issue. Unreal the stuff you uncover on these old cars it never fails to amaze me what someone was thinking way back when they did this. the shop that i brought it too today just happened to have a 4 speed manual brake 69 Mach 1 original car in their shop and this is how the discovery was made.
That explains the relatively low pressure all around, but it doesn't explain the pressure differential between the front and rear brakes.
Andrew
Other then a bad M/C (front seal leaking past into rear area of bore), how is the pedal pressure being applied? With the variance in rear circuit pressure (900 to 1100 psi), it may be that the same pedal pressure isn't being applied each time.
Bob.
andrewb70
04-09-2014, 06:27 AM
Other then a bad M/C (front seal leaking past into rear area of bore), how is the pedal pressure being applied? With the variance in rear circuit pressure (900 to 1100 psi), it may be that the same pedal pressure isn't being applied each time.
Bob.
Bob,
That's an excellent point. I was under the impression that the front and rear pressures were being measured simultaneously, but that's an ASSumption on my part...LOL
If this was my car, I would get 4 gauges and measure all corners together. This will control for any variance in the application of force to the pedal.
Andrew
My shop installed the correct manual brake pedal on the car however the brakes suck even more and are spongy as hell.
The master cylinder pushrod had to be extended over an inch for the correct pedal and now the system is worse off than before.
We removed the master cylinder and bend bled it 3 times. We then bled each caliper and all bleeders 4 times using the motive power bleeder and the phoenix bleeder. There is not a stitch of air coming out but the pedal feels horrible and mushy, it does not go to the floor and it does stop the car but it just doesn't feel right
Any idea what else i can do here? I also notice fine metallic in the master cylinder reservoir fluid which i don't think is normal. Maybe my master cylinder is defective with its seal. With initial pedal pressure you can hear a squish sound coming from the push rod area.
I give up on all this aftermarket crap we get sold these days. Everyone here thinks is the master cylinder as the culprit.
dontlifttoshift
04-12-2014, 10:38 AM
Please hook pressure gauges to both ports at the master cylinder and record pressures.
You said pads are definitely bedded in, did they glaze over? Pads should be BP10s. They don't take a lot of heat to bed in and they stop quite well then. Are the rotors just the way they came from wilwood?
andrewb70
04-12-2014, 10:49 AM
There are Ford OEM 7/8" bore MCs available.
Andrew
Well the verdict is in, defective mc. They removed the lines and plugged both ports and the pedal was still mushy. They took the mc off and the bore is all scored and chewed up which means the fluid was leaking into the rear. This explains all the metallic particles in the reservoir. They are replacing the mc this week. Il report back later in the week with the outcome.
Steve N 69 69 69
07-08-2014, 04:18 PM
So? Did that fix it? I've been wanting to upgrade to those same calipers, so I hope they work great now.
minendrews68
07-12-2014, 10:01 AM
It would've been nice to hear how this ended up, at least for someone else that might have this problem.
It would've been nice to hear how this ended up, at least for someone else that might have this problem.
the main issue was the wilwood master cylinder due to aluminum debris in the bore. what was happening is the rubber o-ring seal was chewed up for the front bias therefore only the rear of the stroke was getting a good seal. Summit replaced the unit under warranty no questions asked. very bad quality control at Wilwood if you want my opinion. the new master cylinder showed up and while bench bleeding was loaded with aluminum particles. i had the unit taken apart and cleaned 4 times carefully and now everything works as designed. i still swapped pads to Carbotech XP8 and AX6 now my car stops great as i originally wanted it to. So for anyone buying these aluminum MC please take then apart and clear carefully before you run into the issues i had, don't trust it out of the box because they are not perfectly cleaned out.
Monitor it, because we've usually seen the alum MC getting scuffed up internally as a geometry of pedal/pushrod/firewall-mount problem.
Powered by vBulletin®