Using a smaller master cylinder on the JB7 brakes (13" drums) ok?

[Mattchu60]

I should start by saying - I upgraded to the JB7 brakes from a 3/4 ton (larger front calipers, rotors and 13" rear drums) 2 years ago on my truck which was originally a half ton. I did not touch the Master Cylinder, just replaced the flexible lines and everything bolted up fine.

The JB7 system came with a larger Master Cylinder and booster vs what my truck currently has. I recently came into possession of the larger MC and booster, but it appears the front/rear fittings on that MC are reversed in position on larger MC vs what I have now. This means to install it, I would either need to replace the lines from the proportioning valve to the MC, or bend the lines to fit the new positions.

So far, I have had no issues running the smaller MC, it seems to work fine. I do wonder though - if GM designed the system and thought a larger one was needed, maybe I should consider getting the proper one on there. Or maybe its pointless to switch them out.

The main difference I can see - the smaller MC has a 1-1/8" bore, the larger one has a 1-1/4" bore.

What do you guys think or know, I know little about brake system design, other than the basic concept of how they work.

[homemade87]

The larger bore will displace more fluid faster . This will result in a harder pedal . If you have a slightly soft pedal now this may be desirable . The larger bore was for the larger caliper capacity .

If you have it in your possession now with out having to purchase it I would try it . You don't have to bend or replace the lines. There is brass adapters that will convert to accommodate the size change . I did this when I was going thru trying to find a master to work with my rear disc conversion . My local parts store had them .

[Mattchu60]

Thanks for input - do the front and rear port displace the same amount of fluid?

I figured they had different diameter ports on the MC so you wouldnt try to switch the spots. Maybe its just because of the line sizes?

[homemade87]

usually the fronts have a larger line because they take more fluid than the rear small cylinder drum brakes . If you have plenty of slack try to see if you can move the lines . Mine had a coil in it and was not to bad to change positions so I could give it a try .

[Keith Seymore]

Originally Posted by Mattchu60

Thanks for input - do the front and rear port displace the same amount of fluid?

During the braking event: it's possible, but the larger reservoir is to accommodate the added fluid needed in reserve during wear of the front discs.

Originally Posted by Mattchu60

I figured they had different diameter ports on the MC so you wouldnt try to switch the spots. Maybe its just because of the line sizes?

Correct - they are "error proofed" to make sure you put the correct line into the correct port.

Originally Posted by Mattchu60

So far, I have had no issues running the smaller MC, it seems to work fine. I do wonder though - if GM designed the system and thought a larger one was needed, maybe I should consider getting the proper one on there. Or maybe its pointless to switch them out.

"The General" has to meet a bunch of very specific test requirements that you don't have to meet: FMVSS 105 and 208, for example, and at various load conditions (curb, 1/2 payload and GVW). If you are happy with the level of braking you are getting, in terms of pedal feel, travel and force vs deceleration, then...you are probably ok.

K

[USSkoval]

I have a Burb that was originally a 1/2 ton but I converted it to a 3/4 ton front and a rear from a 1 ton with the big brakes. I left the stock master in place and it has always worked well for daily driver duties and towing. I've also had a few 3/4 ton Burbs with the proper master cylinder, they have a harder pedal due to the larger master cylinder. I prefer the 1/2 ton master due to it's softer pedal feel... BUT, there is one drawback. You have to make sure the rear brakes are properly adjusted or you end up with excessive pedal travel.

[SCOTI]

Originally Posted by homemade87

The larger bore will displace more fluid faster . This will result in a harder pedal . If you have a slightly soft pedal now this may be desirable . The larger bore was for the larger caliper capacity .

If you have it in your possession now with out having to purchase it I would try it . You don't have to bend or replace the lines. There is brass adapters that will convert to accommodate the size change . I did this when I was going thru trying to find a master to work with my rear disc conversion . My local parts store had them .

I've always understood it as opposite..... The smaller the bore diameter, the higher the pedal pressure. Pro-touring guys use smaller diameter bore MC's to help tune their pedal pressure when running manual big-brake set-ups.

I used a smaller bore MC in my dually to help increase line pressure when I went from the Hydra boost set-up to vacuum brakes.

[homemade87]

Originally Posted by homemade87

usually the fronts have a larger line because they take more fluid than the rear small cylinder drum brakes . If you have plenty of slack try to see if you can move the lines . Mine had a coil in it and was not to bad to change positions so I could give it a try .

Originally Posted by SCOTI

I've always understood it as opposite..... The smaller the bore diameter, the higher the pedal pressure. Pro-touring guys use smaller diameter bore MC's to help tune their pedal pressure when running manual big-brake set-ups.

I used a smaller bore MC in my dually to help increase line pressure when I went from the Hydra boost set-up to vacuum brakes.

What you say is true . I think you miss read my statement . I was talking volume not pressure . Two different things . In your case if you did not have volume to fill the brake system before the pedal could apply pressure it would go to the floor ( to small of bore ) . If the volume was to great ( bigger bore ) the pedal would sit at the top and you would have to stand on it for any kind of braking . The trick is finding something in the middle so the volume can fill the system at lets say 1/3 pedal and when you apply foot pressure to increase the system pressure with out having to stand on it or go to the floor . Thats why you had to go to a smaller bore . Hydro did the work for you with the bigger bore . Now you are having to do the work with a smaller bore with better mechanical advantage .

[SCOTI]

Originally Posted by homemade87

What you say is true . I think you miss read my statement . I was talking volume not pressure . Two different things . In your case if you did not have volume to fill the brake system before the pedal could apply pressure it would go to the floor ( to small of bore ) . If the volume was to great ( bigger bore ) the pedal would sit at the top and you would have to stand on it for any kind of braking . The trick is finding something in the middle so the volume can fill the system at lets say 1/3 pedal and when you apply foot pressure to increase the system pressure with out having to stand on it or go to the floor . Thats why you had to go to a smaller bore . Hydro did the work for you with the bigger bore . Now you are having to do the work with a smaller bore with better mechanical advantage .

I did interpet your statement "the larger bore will displace more fluid faster . This will result in a harder pedal" as meaning pressure.

[USSkoval]

Originally Posted by SCOTI

I did interpet your statement "the larger bore will displace more fluid faster . This will result in a harder pedal" as meaning pressure.

No, you will be required to apply more pedal pressure to achieve the same line pressure. Harder pedal means more work to get the job done.

[Tom]

Larger bore master will result is less pedal effort for same amount of line pressure put to braking system, meaning softer pedal. Hence the reason everyone with an 88-98 puts 99+ master cylinders on for harder pedals, because the 99+ has a smaller piston.

[USSkoval]

Originally Posted by Tom

Larger bore master will result is less pedal effort for same amount of line pressure put to braking system, meaning softer pedal. Hence the reason everyone with an 88-98 puts 99+ master cylinders on for harder pedals, because the 99+ has a smaller piston.

This is not true. Smaller bore provides a softer pedal because it is not moving as much fluid. There is more pedal travel to get the same amount of fluid movement, period. The crap brakes on those trucks is a whole nuther ball of wax and has more to do with the ABS on the later models. The earlier models tend not to have the crappy low brake pedal.

Like I said before, I have personal experience with the exact setup the OP is inquiring about. I have run both master cylinders, the smaller bore provides a softer pedal FEEL and a little more pedal travel.

GM themselves are aware of that fact too. One example is the 3rd gen F-body, the dealers used to install the smaller disc/drum master on the disc/disc cars when the customer's complaint was a hard brake pedal.