NEED SOME INFO ON BRAKE PEDAL RATIO PROBLEM

ARPM

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Jul 8, 2004
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PUERTO RICO
After a few years of my stroke i am back in to my favorite theraphy.. My 65 vert project! 3 years have pass since the begining and i have enjoy watching my body man,and son build up my stang one stept at a time. 90% of body/compact metal was replaced and a beutifull signal flare red color covers it now.

While the brakes were been installed a small problem came up and still i haven't figure a way to solve it.

First some info on what's on the car brake system.

Front original Kelsey Hayes 4 piston calipers,

Russell braided lines in front and rear,

Brand New Rear Crown Victoria calipers attached with 1993 Toyota Corolla front caliper brake hoses and custom build Stainless Steel Hose Brackets,

Custom build parking brake cable and adjuster, MBM 7" single diafragm booster with custom made fire wall braket and custom made adjustable push rod,

MBM Chrome Aluminum 1.0" bore dual master Cyl. and MBM Combination Valve (GM Style) for 4 wheel disc brakes.

All metal lines are 3/16" stainless steel and i have installed one man Russell brake bleeders in the front Kelsey Hayes calipers.

The problem is that after bleeding the system (Bled with Snap-On pressure tank) i have a very long travel in the pedal before it starts to engage. Pedal distance from the floor is 6 3/4" while in my 66 GT is only 4" but the pedal rest at the rubber bumper in the hanger assembly.

I was told that increasing the master cyl bore to 1 1/8" will correct this but i am not to sure about it since it's almost 3" of additional travel that have to be eliminated.

Another guy said that by moving the clevis pin on the pedal down will cause the ratio to be corrected but this is imposible because the booster rod is the screw type hat can't be bended down or up to compensate for the new angle.

Re-using the old booster bracket is also imposible because the total lenght of the bracket/booster/master cyl is so long that it hit's the shock tower.

So my dilema is what route to go and if any one here has change the bore of the master cyl. , what has been the total distance shortened by the change and what was the bore size used? I saw Mustang Steve Clevis Pin Mod but is for the flat push rod that moves up and down. Mine is the round threaded rod without the swing arm in the bracket.

On the other hand the 1992 Crown Victoria Calipers were installed and one was looking towards the front and the other to the rear this is because the same part number is used on both brackets for left and right. I decided to invert the bracket in the passenger side so both caliper were set in the rear but at the expense of having to remove that caliper at bleeding time due to the fact that the bleeder screw sits at the bottom in the installed position. Has any one here done the Crown Victoria Conversion and tell me what was the final caliper positions?

Any sugestions or opinions are gladly appreciated
As allways thanks in advance for all your great help and assistance.

Andrew
 
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Andrew,

I'm thinking you have one of two possible issues. Either you've got some air in your master cylinder or you may need to add a residual pressure valve into the rear brake line.

If you've got a brake proportioning valve (and you should with a rear disc swap),
- dial out the rear brakes completely and check your pedal travel. If you have a firm pedal (as you are now only activating the front, stock Kelsey Hayes calipers), then the issue is with the rear brake system. If you still have a long pedal stroke, then the issue is probably due to air in the Master Cylinder itself.

If you have air in the Master Cylinder, you need to 'Bench Bleed the Master Cylinder'.

If you have isolated the problem to the rear brakes, then you may need to add a residual pressure valve into the rear brake line.

Proportioning Valve - http://www.autoanything.com/brakes/61A3155A0A0.aspx
Residual Pressure Valve - http://www.autoanything.com/brakes/65A3153A0A0.aspx
Bench Bleeding Master Cylinder - http://www.misterfixit.com/brakbld1.htm

At least it's some place to start. Good luck and keep us informend.

Tim
 
Andrew,

I'm thinking you have one of two possible issues. Either you've got some air in your master cylinder or you may need to add a residual pressure valve into the rear brake line.

If you've got a brake proportioning valve (and you should with a rear disc swap),
- dial out the rear brakes completely and check your pedal travel. If you have a firm pedal (as you are now only activating the front, stock Kelsey Hayes calipers), then the issue is with the rear brake system. If you still have a long pedal stroke, then the issue is probably due to air in the Master Cylinder itself.

If you have air in the Master Cylinder, you need to 'Bench Bleed the Master Cylinder'.

If you have isolated the problem to the rear brakes, then you may need to add a residual pressure valve into the rear brake line.

Proportioning Valve - http://www.autoanything.com/brakes/61A3155A0A0.aspx
Residual Pressure Valve - http://www.autoanything.com/brakes/65A3153A0A0.aspx
Bench Bleeding Master Cylinder - http://www.misterfixit.com/brakbld1.htm

At least it's some place to start. Good luck and keep us informend.

Tim

65 Fastback, Thanks for the reply. The master cyl was bench bled before installation and 2 more times after the installation. The whole system was pressure bled using the Snap On pressure bleeder tank 2 times ( This replaces the entire fluid in the system with fresh fluid by pressure pumping fresh fluid into the master). The Combination valve from MBM has the needed valves for disc-disc setup. The system has no leaks anywere and every single component and lines are brand new.

In one of my attemps to fix the problem i got me a new Bendix 11944 master cyl with a bore size of 1 1/8". This master is for GM applications 68-74 Disc-Drum so the rear integrated residual valve was removed to modify it for disc-disc and a temporary set of lines were hooked to the combination valve for testing. The pedal travel improved about 1 " less. Still i need about 2" less travel. But there are a few additional problems with this unit.

1st: there is not a chrome cover for it anywere (wierd size) even do it looks like the dual master from 1967-68 Ford it's wider and longer,

2nd: the reservoirs are reverse from my installtion requiring new stainless steel lines

3rd the rear reservoir it's very small to supply the 2 rear crown Vic's calipers.

Looks like my only solution is to get the MBM version of the one i have but in 1 1/8" bore. but it will be a $175.00 for that unit and i still have the one wrongly sent to me that cost me $160.00 and still it will be a trial and error test.

So i am still looking for recomendetions from any one that has gone thru this problem and has changed the master cyl. for a bigger bore one.
 
I"ll throw in my 2 cents. The bigger the bore on the master cylinder, the harder it is going to be to push. "Pascal's Law". 1 1/8" pistons push more fluid but at a lower pressure (as compared to a 7/8 piston about 22.7% less with the same amout of pedal pressure). A 7/8 will give higher pressure but less volume. With those 4 piston calipers, its a must to have a large resivor of fluid or you could run out. If not, you need to check the fluid more often as the pads wear down and the level will drop quicker. This is when the bowl size comes into play. This is also true on rear disc also. If the rear caliper is mounted upside down, then there will be air in it resulting in compressability (soft pedal and more travel) air is easily compressable, liquids are not. No way to remove it. On lowering rod pin, bad idea, and bending the push rod will cause it to bow and fail under extreme pedal pressure. On the Vics rear disc, both bleeders need to be up. One should mount to the back and the other to the front. Even if you take it off to bleed, there is still a possibility that air can re-enter the caliper. I would also use a adjustable poportioning valve to adjust the rear brakes. Also check to see if the pushrod stays in if you pull the pedal all the way back, (if you catch your foot under it, the rod could come out, then NO BRAKES!) Heres a couple of possibilities.
Bad master cylinder (bypassing)
Air in system (my guess)
Hoses can ballooning requiring more fluid volume (more pedal travel)
Bleed system the ole fashion way
 
Andrew,

Did you try dialing out your rear brakes with your proportioning valve? At least you can isolate the issue to the MC or the rear brake system.

My guess is the problem will be in the rear brake system. Single piston floating calipers require more displacement then 4 piston fixed calipers, so your rear system is going to require more pedal travel. I think your 1-1/8" MC should be sufficient. You should still look into the residual pressure valve, they are different for rear drum and rear disc systems. A residual pressure valve for a drum system is 10psi, while the rear disc system is 2psi.

Also, Evil Eleanor makes a good point on the mounting of your passenger side rear caliper. You may need to unbolt it and bleed it while it is right side up, then reinstall.

Keep us updated.

Tim
 
I am running Granada discs up front and discs in the rear. I just used a mc from a mis 80's Granada power brake car, not hydro assist, and it seems to work fine. I picked up a rebuilt one for around $30. In addition, I am running it on a booster from an early 90's V8 Mustang. The rear bowl is smaller than the front, but I think most are. (Someone correct me if this is wrong.) Pick up a chrome cover for another $10 and you are rolling........ or at least stopping.
 
SOLVED THE PEDAL RATIO PROBLEM

Ok, this is what solved the pedal ratio problem. After testing the 1.125" GM Master Cyl. (Bendix 11944) there was a reduction of aprox. 1.5" in pedal travel. This Master Cyl is 8.0" Long (Same size as the MBM one). The rear reservoir was too small to keep the Crown Vic's calipers full at all time not to mention that when brake pads and rotors wore out that would even get worst. Then the unit was not chrome so required a chrome job and the inverse position of the lines made it close to a no no for the project. Even finding a chrome reservoir cap was a challenge. I can't find a single one anywhere in the net.

So searching around I found a master cyl from GM called the Moraine type that was 7" long but had taller reservoirs compensating the lack of fluid to the rear crown Vic's calipers. The new shorter master cyl had less stroke on the pedal (7" vs. 8") but it had the 1.125 bore size that will pump more fluid and required less pedal travel. The only mod to be done was the removal of the rear residual valve because it was designed for a Disc/Drum application. I found a brand new one locally for $42.00 so it was low cost for trial and error test and in the event it didn't work as expected the Master Cyl will fit my son's 69 Camaro.

Results were, pedal travel decrease by 2.89", Stopping power it's excellent due to fact that is helped by a 7" MBM Booster and heck I gained a 1" additional space at the engine bay making it possible to install a Dual Diaphragm 7" Booster that is about an inch longer than the single one I have.

This morning I ran to my chrome shop (They are my customers at My Computer Store so I get VIP Treatment) and had it chromed by 3:00pm along with the fittings and adapters.{9/16"-18 to 3/16 tube(3/8"-24) and 1/2"-20 to 3/16" tube(3/8"-24)} cuz my other master took the 3/16" SS lines directly without adapters. The Stainless lines had 2 loops at the connection to the combination valve so stretching one and shortening the other was a snap.

To remedy the Crown Vic's right rear up side down caliper i had my local machine shop drill and tap a new bleeder at the actual position and plugging a cap cover at the old bleeder hole.

Here is a Pic of the new Master Cyl and the old one installed in the car.
 

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Thanks for the update, it always helps when someone returns to a post to update it as to what corrected the initial question.

...The rear reservoir was too small to keep the Crown Vic's calipers full at all time not to mention that when brake pads and rotors wore out that would even get worst...

BTW, a $10 disc brake residual pressure valve would have produced the same outcome with the original MC.

Tim
 
Thanks for the update, it always helps when someone returns to a post to update it as to what corrected the initial question.



BTW, a $10 disc brake residual pressure valve would have produced the same outcome with the original MC.

Tim

The 10# residual was tested in the rear setup. While the pedal travel didn't reduce i notice that at the 1st pedal pump it had less travel but not more than 3/4" the second time if done quickly no pedal decrease at all. Also tried a 2# but that one didn't change anything at all.