46 Ford Build Fails Brke Plate Test With High Pedal Pressure

[Sedandelivery]

Submitted my 46 Merc for an Engineer required Brake Plate Test, MK1 Jag front end, NF Fairlane disc braked diff with NF master cylinder and booster and the tester says the pedal pressure is to high. Brakes work but pedal pressure is 10% outside limits.
Anybody been in a similar experience? How did you get out of it? Cheers!

[jeffa]

Are they saying you need to press the pedal too hard to stop the car, or that there's too much pressure and the potential of easy lock up?
Either way, what pedal setup are you using? Does it have the same ratio as the brake parts' donor car?
Just a thought.

[Sedandelivery]

Thanks for your reply.....I find this process a little confusing, I assume there's a specification that's worked to and my vehicle requires to much (10% to much) pedal pressure to stop. I've driven this vehicle and it seemed unremakable in it's pedal pressure but it doesn't meet the specification it seems. The pedal assembly and diff are from the same donor vehicle, the Jag front end isn't, because the Jag front end is such a common conversion I expected few issues. When I get it back from the test station I'll do a brake ratio calculation and see if it fits within the desired range. I'm surprised the test mechanic couldn't offer any advice on what was required other than maybe there's an issue with the pedal ratio. To get it back and forth to the test station costs $500 so I can't afford to many of those trips.

[FRANK BASILE]

Pedal ratio or go to a smaller bore master cylinder .

[FRANK BASILE]

Pedal ratio or go to a smaller bore master cylinder .

Starting from scratch. The pedal box/master/booster are from the same vehicle as is the rear brakes? . What discs are up front? Jag 2 spot or 4 spot ? . The only change I can see is those front calipers. A smaller bore master cylinder should help with reducing pedal effort [ better fluid multiplication using the law of hydraulics] Pretty sure the Ford would be a 1" master cyl ? Try 7/8" . .

[Sedandelivery]

I did the ratio calculations today and the current set up is about 4 ......everything is matched other than Jag front calipers......according to my maths I need a pedal ratio of 7.5..........can't get that by playing with pedal geometry......I'll recalculate it with a 7/8 master cylinder!

[Gojeep]

Remember that the smaller the master the less pedal pressure but the longer the travel. If you are not using much travel, you only get benefits from a smaller master.
Anyway of moving the pivot point of the master attachment point up closer to the pivot point of the pedal? Or making the pedal longer?
4 is way too low a ratio.

[FRANK BASILE]

Remember that the smaller the master the less pedal pressure but the longer the travel. If you are not using much travel, you only get benefits from a smaller master.
Anyway of moving the pivot point of the master attachment point up closer to the pivot point of the pedal? Or making the pedal longer?
4 is way too low a ratio.

.

I missed that point. After a re read I must assume that he has not used the pedal box assembly from the NA. That should not have been a problem. Agreed that the 4:1 is too low.

[Sedandelivery]

Here's my calculations......caliper piston area 5.066mm2.....master cylinder piston area 0.79mm2...divide 0.79 into 5.066 gives you 6.4 required pedal ratio.....so alter the pedal arrangements to give a 13'' lever with a master cylinder push rod 2'' from the lever pivot....divide 13 by 2 giving a pedal ratio of 6.5......that's got to be close enough....I've done a mock up trial with the modified pedal arrangements which appears to reduce the pedal pressure considerably but I can't actually measure the force required. Have I made an obvious error?

[Sedandelivery]

I made the obvioius error myself there...those piston area figures are square inches not mm 5.066 square inches and 0.79 square inches

[32V8]

And your engine vacuum is what?
If you are using OEM calliper's, pedal box, master cylinder etc, the problem is not there. I can not see anything about vacuum measurement, this is critical for having light pedal pressure.
You have done the right thing using ALL brake components from the same car, its when you mix and match you see problems.

[Harv]

And your engine vacuum is what?
If you are using OEM calliper's, pedal box, master cylinder etc, the problem is not there. I can not see anything about vacuum measurement, this is critical for having light pedal pressure.
You have done the right thing using ALL brake components from the same car, its when you mix and match you see problems.

Suspect the engine would need to be lumpy as all hell to cause that much angst (<10"Hg) given the Fairlane booster diaphragms are a decent diameter. Vacuum pressure has a much greater impact on recharge time for the booster between brake applications.

Cheers,
Harv

[32V8]

Suspect the engine would need to be lumpy as all hell to cause that much angst (<10"Hg) given the Fairlane booster diaphragms are a decent diameter. Vacuum pressure has a much greater impact on recharge time for the booster between brake applications.
Cheers,
Harv[/quote]

We had problems with a warm SBF, which was supposed to have "good vacuum". After months of trying to get good brakes someone suggested "have you checked what vacuum you have"?
After we did we then knew the problem, so check your vacuum.
Also check you are taking full manifold vacuum and not off a runner.

[Sedandelivery]

We did check manifold vacuum, casually, at the booster plus checked the operation of the booster and it conforms to the Ford workshop manual specifications......after your post we actually put a gauge on the vacuum line and it was pulling 19 inches at idle so we were satisfied with that. Thanks for your comments, Cheers!

[Chrisso]

Thanks for your reply.....I find this process a little confusing, I assume there's a specification that's worked to and my vehicle requires to much (10% to much) pedal pressure to stop. I've driven this vehicle and it seemed unremakable in it's pedal pressure but it doesn't meet the specification it seems. The pedal assembly and diff are from the same donor vehicle, the Jag front end isn't, because the Jag front end is such a common conversion I expected few issues. When I get it back from the test station I'll do a brake ratio calculation and see if it fits within the desired range. I'm surprised the test mechanic couldn't offer any advice on what was required other than maybe there's an issue with the pedal ratio. To get it back and forth to the test station costs $500 so I can't afford to many of those trips.

How much for a plate test??????? If I read it correctly it was $500.00 Jesus H Christ I paid $50.00 down here in Albury and that was because the test was $20.00 and all I had was a 50 and he had no change. Does yours look like this?? Don't worry about the the drag alignment test solid axle vehicles fail this.