Front Calipers Leaking! What to Replace Them With???

[70_Steve]

Like the title says...

I had been thinking of using this as an opportunity to "upgrade" to the T-bird calipers. Since installing my hydroboost, it's been very apparent that my stock rear drum brakes are locking up before the fronts, which are stock 76 Dana 44 disks. The eventual plan is to upgrade the rear to Explorer disks but the driver's front caliper decided to start leaking now...

To help make the decision, does anyone know the diameters of the stock 76 caliper piston vs the diameter of the T-bird pistons? I'm thinking that if the combined area of the T-bird pistons is significantly larger, then installing the T-bird calipers will only make my current brake bias problem worse. Yes, I am planning on installing an adjustable proportioning valve at some point. Maybe just do that now and get it over with?

I currently have the stock 76 proportioning valve installed. If I install the adjustable valve, should I simply remove the stock valve??

 

[Apogee]

The F150/Bronco calipers have 2.88" diameter pistons, so an effective piston area of about 6.5 square inches. The T-Bird/LTD/etc calipers have 3.125" diameter pistons with an effective area of 7.67 square inches, so an 18% increase in front caliper piston area. This will increase your front brake bias, which will make front lockup occur sooner, which if you're experiencing premature rear lockup, would be a step in the correct direction, right?

As for the proportioning valve, you never want more than one PV in the rear brake circuit as the result isn't linear or predictable, so if you want to install an aftermarket adjustable unit, then remove the OE combination valve.

 

[PaveBronco]

Ditch the proportioning valve, run straight pressure to the front and adjustable on the rears, that way you can fine tune to your specific rig.

 

[70_Steve]

The F150/Bronco calipers have 2.88" diameter pistons, so an effective piston area of about 6.5 square inches. The T-Bird/LTD/etc calipers have 3.125" diameter pistons with an effective area of 7.67 square inches, so an 18% increase in front caliper piston area. This will increase your front brake bias, which will make front lockup occur sooner, which if you're experiencing premature rear lockup, would be a step in the correct direction, right?

Thanks for the info, and for confirming what I already suspected. And that is the total area of the Tbird pistons is greater. But the part I wasn't sure of is the overall effect it would have on breaking, given my current combo.

My initial thought was that it would take a larger volume of fluid to move the T-bird pistons, and the pad, the same distance as the stock calipers. The extra movement of the master cylinder to move the front brake pad, would try to shove more fluid to the rear brakes, and make the problem worse.

I guess I don't understand when the effect of moving the piston(s) a certain distance is offset by the difference in pad pressure from the larger piston area...

As for the proportioning valve, you never want more than one PV in the rear brake circuit as the result isn't linear or predictable, so if you want to install an aftermarket adjustable unit, then remove the OE combination valve.

OK, kinda figured. Thanks for confirming...

 

[toddz69]

My initial thought was that it would take a larger volume of fluid to move the T-bird pistons, and the pad, the same distance as the stock calipers. The extra movement of the master cylinder to move the front brake pad, would try to shove more fluid to the rear brakes, and make the problem worse.

It will take a larger volume of fluid to move the pistons but you're exerting the same pressure in the lines with your foot and the hydroboost assist, so it's not forcing more fluid or pressure to the rear brakes.

You may see a slight decrease in pedal height when the brakes are applied, but it will likely be negligible. I can't recall if you used the 1 1/16" bore Dodge MC or the 1 1/8". I use the 1 1/8" and didn't notice much pedal height change between the Chevy calipers and the T-Bird calipers.

Todd Z.

 

[70_Steve]

It will take a larger volume of fluid to move the pistons but you're exerting the same pressure in the lines with your foot and the hydroboost assist, so it's not forcing more fluid or pressure to the rear brakes.

You may see a slight decrease in pedal height when the brakes are applied, but it will likely be negligible. I can't recall if you used the 1 1/16" bore Dodge MC or the 1 1/8". I use the 1 1/8" and didn't notice much pedal height change between the Chevy calipers and the T-Bird calipers.

Todd Z.

Thanks for the explanation Todd!

FWIW, I'm using the 1 1/16" Dodge MC. But I've got great pedal height and feel now, so I should be OK.

Did you notice a difference or change in brake bias when you installed the t-bird calipers?

 

[cldonley]

The advantage of the larger caliper piston area is that the pressure exerted by your foot is multiplied by the area of the piston. Therefore, if the pistons are 18% larger, the same effort results in 18% more braking power in the front with no increase in the rear (providing there is no air in the system). Your pedal travel may change slightly, but remember the larger piston travels the same distance as the smaller one, so as long as there is no air in the lines, you get lots more braking power with the same effort.

 

[toddz69]

Did you notice a difference or change in brake bias when you installed the t-bird calipers?

Not much. It's generally been dialed in so nicely that I haven't even had to mess with my adjustable proportioning valve (it's all the way open). However, I recently noticed in the dirt that my rears seem to be locking a bit prematurely so I may dial in the knob a bit.

Todd Z.

 

[73azbronco]

I'm finishing this mod, one question, it looks like the bleed valve on the Tbird caliper is lower than the banjo hose fitting, or is this an optical illusion. The reason I ask is how do you bleed them if the bleed screw is lower than the hose inlet,,,do I need to remove the caliper,, bleed, then put the. Caliper back on?

 

[toddz69]

I'm finishing this mod, one question, it looks like the bleed valve on the Tbird caliper is lower than the banjo hose fitting, or is this an optical illusion. The reason I ask is how do you bleed them if the bleed screw is lower than the hose inlet,,,do I need to remove the caliper,, bleed, then put the. Caliper back on?

No need to remove the caliper for bleeding.

Todd Z.

 

[Apogee]

It isn't the location of the bleed screw so much as it's the location of the intersection of the bleed screw port and the piston cavity/bore.

 

[jckkys]

This thread contains great information. My '77 also tends to lock up the rear brakes on hard stops, so larger diameter caliper pistons would help. Experience with intermediate and full size Ford, Lincoln, and Mercury cars from the late '60s-'78 showed me they had similar or identicle front disc brake calipers. What models and years have the larger 3 1/8" diameter caliper piston? By describing a part as being T-bird, a huge selection of good potential donors is overlooked. Junk yards have part interchangeability software that I always wanted but they don't let you use.

 

[chuzie]

Functioning brakes; must be nice. ;-)

 

[toddz69]

This thread contains great information. My '77 also tends to lock up the rear brakes on hard stops, so larger diameter caliper pistons would help. Experience with intermediate and full size Ford, Lincoln, and Mercury cars from the late '60s-'78 showed me they had similar or identicle front disc brake calipers. What models and years have the larger 3 1/8" diameter caliper piston? By describing a part as being T-bird, a huge selection of good potential donors is overlooked. Junk yards have part interchangeability software that I always wanted but they don't let you use.

Back in the day when I discovered this upgrade, I went to the library and researched all the models with the larger calipers in the Hollander Interchange Manual and posted it on EBML/B'Fix (I don't think classic existed yet). I had enough people ask me about the makes and models and I could never remember them all so I just started telling people to get calipers from a '76 T-Bird, which I knew was one of the models that had them.

My reasoning for remembering just one model was that we weren't going to go get used calipers at the junkyards anyway. We'd go to the auto parts store and get rebuilds - the junkyards typically charge more for calipers now than the core charges so it didn't make any sense to me to scour the yards.

In more recent times, I think Chuck from BCBroncos has posted the list of all the models with the larger calipers - I know he kept the list I posted way back when.

Todd Z.