Need help with rear disc swap

[BigRed76]

I'm looking for some insight here with my brake situation. I'll start with the details...

Front brakes:
Stock 71 11" rotors
Stock 71 2.935" Calipers

Rear brakes:
Astro van 11" rotors
Stock 71 2.935" calipers (same as front)

Stock 76 corvette master cylinder 1.125" bore
Wilwood adjustable proportioning valve (fully open at this time)

When I initially did the rear disc swap I was using the 76-78 Eldorado calipers (2.497" bore), and for the life of me could not get a firm pedal. The braking power was excellent, but the pedal was always a little spongy, even with the e-brake applied slightly. I then decided to put on some regular calipers, so I went with the same calipers that were on the front. Now the pedal feel is even worse. The truck has enough braking power to stop, but in a hard stop the pedal goes to the floor. I have manually bled and vacuum bled the system with no improvement. Seeing how the pedal feel has gotten worse with bigger rear calipers, I'm suspecting this corvette master cylinder is too small. Everything I have read about master cylinder sizing with 4 wheel discs seems to be a dead end as far as the end result. Any opinions on my setup would be appreciated. Thanks

[chevy_mike]

Do you have a power booster? If not, your MC size is way too big. Manual brakes bore are almost always 1" or smaller.

[BigRed76]

It does have the stock booster, it's the bigger one I believe 11".

[Keith Seymore]

Sounds like the rear calipers are way too big to me.

Normally rear calipers are quite small in comparison, since most of the braking is done with the front. My guess is you are trying to displace too much fluid with those "big" calipers on the rear.

Also - you say "Astro Van" rotors? Do you mean like a Chevy "M" van? I could be wrong but I remember all of those being a rear drum application.

K

[BigRed76]

The rear calipers are a bit bigger than I'd like, but I wasn't able to find a D52 caliper with a smaller piston, for a stock application. I'm not really wanting to spend big bucks on the rear calipers if I don't have to.

As far as the Astro van rotors, they are the front rotors off of the awd Astro/safari vans. They are I slip on rotor with the 5x5 bolt pattern, and they are the rotors used with captainfab's rear disc brake brackets.

[BigRed76]

I've been researching other caliper options tonight, and I've found Wilwood's "GM III" caliper line, it's a direct replacement caliper for the GM D52 calipers, and they are available with piston sizes of 1.75", 2.38", and 2.75". I'm thinking the 2.38" calipers would be a good size for the rear, and they can be had for about $90 a piece. I'm still open to opinions as I'm not dead set on these calipers yet.

[VictoriaHardware]

This site has a ton of excellent info- http://www.pirate4x4.com/tech/billavista/Brakes/

About 3/4 of the way down the page is a downloadable spreadsheet. You can put all of your variables in and get a good idea how your truck's brake system will work before you ever buy the first part.

I followed it and recently completed a 4 wheel disc conversion on my '66 C10. I too used stock GM truck front calipers, Corvette 1.125" bore master, and Eldo rear calipers. I too had an awful time with the rears but finally got it. Now the whole system works very well. Put in your info and the spreadsheet will convince you the front caliper and master you have are correctly sized. It will also convince you the Astro rear calipers are too big diameter-wise.

[knomadd]

Check out wilwoods FAQ (http://www.wilwood.com/TechTip/TechFaqs.aspx#MC) and scroll down to the master cylinder section if the link above doesn't take you there automatically. There's also a section on pedal pressure. Some of the tips in here might help.

It sounds like you still have air in the system, or the master is faulty or isn't correct. But I'm just guessing based on what you've said about it.

[lolife99]

Wilwood sells 2 different D52 calipers for exactly what you are trying to accomplish.
Large diameter piston in the front calipers,... and a smaller diameter piston for the rear calipers.

[picklito]

I agree you need a way smaller rear caliper. And better bleeding.

[BigRed76]

I spent some more time bleeding the brakes this weekend, with no improvement of course. I did pinch off the rear brake hose to the rear axle, and the pedal became rock hard (with engine off) verifying for me that the rear is definitely the issue.

[picklito]

You may have an air bubble that is trapped in a line or hose, often at the top of a bend. Vacuum bleeding may help.

[BigRed76]

Originally Posted by VictoriaHardware

This site has a ton of excellent info- http://www.pirate4x4.com/tech/billavista/Brakes/

About 3/4 of the way down the page is a downloadable spreadsheet. You can put all of your variables in and get a good idea how your truck's brake system will work before you ever buy the first part.

I followed it and recently completed a 4 wheel disc conversion on my '66 C10. I too used stock GM truck front calipers, Corvette 1.125" bore master, and Eldo rear calipers. I too had an awful time with the rears but finally got it. Now the whole system works very well. Put in your info and the spreadsheet will convince you the front caliper and master you have are correctly sized. It will also convince you the Astro rear calipers are too big diameter-wise.

I read this entire thread, and it was by far the most helpful brake info I've found. I did the spread sheet with my info, and it did confirm that the 2.935" calipers are too big for the rear. I entered a 2.38" rear caliper in the spreadsheet and that seemed to work much better. I was incorrect about the rotor sizes, I double checked and the stock 71 front rotors are 12" and the astro van rotors I have on the rear are 11.5". And I did not see anywhere in the spread sheet where the rear rotor size would cause an issue, but either way those are the rotors I have to use with CaptainFab's disc brake brackets.

With that being said I think I'm going to go ahead and order the Wilwood GM III calipers with the 2.38" pistons.
http://www.summitracing.com/parts/wi...5344/overview/

[BigRed76]

Originally Posted by picklito

You may have an air bubble that is trapped in a line or hose, often at the top of a bend. Vacuum bleeding may help.

I tried manual bleeding as well as vacuum bleeding, and I am no longer getting any air out of the system. I also tried attaching a hose to the bleeder and putting the other end of the hose into a container of brake fluid, and pumping the pedal but it doesn't even push enough fluid out of the rear calipers to reach the container.

[BigRed76]

Originally Posted by VictoriaHardware

This site has a ton of excellent info- http://www.pirate4x4.com/tech/billavista/Brakes/

About 3/4 of the way down the page is a downloadable spreadsheet. You can put all of your variables in and get a good idea how your truck's brake system will work before you ever buy the first part.

I followed it and recently completed a 4 wheel disc conversion on my '66 C10. I too used stock GM truck front calipers, Corvette 1.125" bore master, and Eldo rear calipers. I too had an awful time with the rears but finally got it. Now the whole system works very well. Put in your info and the spreadsheet will convince you the front caliper and master you have are correctly sized. It will also convince you the Astro rear calipers are too big diameter-wise.

What rear rotors did you use on your '66?

[picklito]

Originally Posted by BigRed76

I tried manual bleeding as well as vacuum bleeding, and I am no longer getting any air out of the system. I also tried attaching a hose to the bleeder and putting the other end of the hose into a container of brake fluid, and pumping the pedal but it doesn't even push enough fluid out of the rear calipers to reach the container.

Well that's a pretty fair bleeding effort. But man it can be hard to get the air out of any high spots. Weird though... despite having a probably too large of a rear caliper, I'm surprised that it's not displacing much fluid to the back. I wonder if that's telling you something?

BTW - I'm following your thread cuz I'm about to do the same sort of swap. Still on 4-wheel drums!

[VictoriaHardware]

Originally Posted by BigRed76

I tried manual bleeding as well as vacuum bleeding, and I am no longer getting any air out of the system. I also tried attaching a hose to the bleeder and putting the other end of the hose into a container of brake fluid, and pumping the pedal but it doesn't even push enough fluid out of the rear calipers to reach the container.

One problem I had sounds similar to yours. It turned out I did not have the intermediate push rod adjusted properly. The rod between the booster and master cylinder was too short, so there was 1/4" too much slop. It is supposed to fill the distance exactly. I had to do a lot of careful measuring, drawing, and math to get it right. So the master cyinder pistons weren't moving far enough. I had very weak pressure to the rears.

My rear rotors are from a late model 3/4T GM truck. They're about 12" dia. I'm using 6 lug wheels.

Glad that link helped. I wouldn't have guessed the a 4x4 site would have that kind of spot-on info that applies to all vehicles. I think someone else on this forum pointed me there.

After getting the booster rod right I still had problems. It wasn't until I fabbed up some plumbing and put gauges in the front and rear lines that I could see exactly what was going on with the pressures. Finally I was convinced that things were correct and the numbers were consistent with the spreadsheet predictions.

[BigRed76]

I might go ahead and unbolt the master cylinder to double check the pushrod length. I do remember before I bolted on the corvette master I compared it to my original master and the holes were the same depth so I never put much thought into it.

As for the calipers I ordered the Wilwood GM III 2.38" calipers so they should be here by the weekend.

[BigRed76]

Update time. I spent some more time on the truck this weekend. I installed the Wilwood 2.38" calipers, using the my existing D52 style pads and the same astro van rotors. After the initial install I gravity bled the rears, then manually bled them to surprisingly find that the pedal still goes to the floor with vacuum assist. I then bled the fronts just to be safe, with no change. I then removed the master cylinder and bench bled it again, which resulted in no change after reinstalling it and bleeding all 4 wheels again. I pinched off the rear axle brake hose and then had a firm pedal, with vacuum assist. So after carefully double checking everything on the rear, and bleeding them again, I noticed that the right rear caliper was moving a considerable amount when the pedal was pressed, whereas the left rear caliper had no noticeable movement. After looking closer at the right rear I found that the caliper and pads are not parallel with the rotor. On top of that the right rear axle seal is leaking...again. That seal has now been replaced twice, and the truck hasn't been driven more than 5 miles since I put in the last seal. So after verifying that the caliper brackets are fully seated against the right rear axle flange, and the rotor is fully seated against the axle, I'm thinking my axle housing may be bent.

So to ensure that my pedal feel issue was related to the pads not being parallel with the rotors I removed the right rear caliper bolts and just let the caliper sit in the bracket so that when the pedal was pressed it would allow the pads to apply parallel against the rotor. Doing this made the pedal considerably stiffer as I thought it would.

Now, I have no idea how the axle housing could've gotten bent, but I do know that right rear axle seal has been leaking since I got the truck 3 years ago. So I think for now i'll try to find another 12 bolt 3.73 axle, although eventually id like to swap in a 9".

[PGSigns]

Here is a problem we run into on Harleys with new calipers. The displacement if the master cylinder is to low to pump up the calipers. They pus enough fluid in to move the pad out some but it pulls right back in and you get no where. So what we do with these is take an air hose and push the pistons out once it is installed on the bike and seat the pads and align the caliper. Then when we bleed it they will work. May be all you need to do on yours. I would make sure the rotors are seating on the axle flange correctly before assuming it is a housing issue. The run out on the rotor and axle flange can be checked easily with a dial indicator. Also if you are having leaks just sitting with a new seal take a good look at the axle and bearing.
Jimmy

[BigRed76]

Originally Posted by PGSigns

Here is a problem we run into on Harleys with new calipers. The displacement if the master cylinder is to low to pump up the calipers. They pus enough fluid in to move the pad out some but it pulls right back in and you get no where. So what we do with these is take an air hose and push the pistons out once it is installed on the bike and seat the pads and align the caliper. Then when we bleed it they will work. May be all you need to do on yours. I would make sure the rotors are seating on the axle flange correctly before assuming it is a housing issue. The run out on the rotor and axle flange can be checked easily with a dial indicator. Also if you are having leaks just sitting with a new seal take a good look at the axle and bearing.
Jimmy

I know what you mean about getting the pistons to extend far enough because it took quite a few pumps of the pedal for them to stay out. And on the left side the pads are all the way against the rotor, but on the right side on each pad there is at least 1/16" difference from the inside edge of the pad to the outside, and vise versa on the other pad.

As far as the rotors being fully seated against the axle I know they are because I tightened up the lug nuts to hold the rotors on, and when you spin the rotor the gap between the crooked pads and the rotor does not change. I also didn't feel the need to check runout since the rotors are new and I have no pulsation.

[crakarjax]

Originally Posted by BigRed76

Update time. I spent some more time on the truck this weekend. I installed the Wilwood 2.38" calipers, using the my existing D52 style pads and the same astro van rotors. After the initial install I gravity bled the rears, then manually bled them to surprisingly find that the pedal still goes to the floor with vacuum assist. I then bled the fronts just to be safe, with no change. I then removed the master cylinder and bench bled it again, which resulted in no change after reinstalling it and bleeding all 4 wheels again. I pinched off the rear axle brake hose and then had a firm pedal, with vacuum assist. So after carefully double checking everything on the rear, and bleeding them again, I noticed that the right rear caliper was moving a considerable amount when the pedal was pressed, whereas the left rear caliper had no noticeable movement. After looking closer at the right rear I found that the caliper and pads are not parallel with the rotor. On top of that the right rear axle seal is leaking...again. That seal has now been replaced twice, and the truck hasn't been driven more than 5 miles since I put in the last seal. So after verifying that the caliper brackets are fully seated against the right rear axle flange, and the rotor is fully seated against the axle, I'm thinking my axle housing may be bent.

So to ensure that my pedal feel issue was related to the pads not being parallel with the rotors I removed the right rear caliper bolts and just let the caliper sit in the bracket so that when the pedal was pressed it would allow the pads to apply parallel against the rotor. Doing this made the pedal considerably stiffer as I thought it would.

Now, I have no idea how the axle housing could've gotten bent, but I do know that right rear axle seal has been leaking since I got the truck 3 years ago. So I think for now i'll try to find another 12 bolt 3.73 axle, although eventually id like to swap in a 9".

What inlet do the GM iii calipers use and how would one connect them to a banjo brake line?

[BigRed76]

The Wilwood GMIII calipers use a 1/8" NPT inlet. I ended up buying new steel braided brake hoses and 90 degree fittings to adapt the hoses to the caliper. I was unable to find a good way to connect a banjo style hose to the calipers.