Yes, the 2002 has discs in the rear.

I did some 60-0 panic stops both before and after the swap, and from what I can tell the rear was locking up. ABS was going crazy, so perhaps a better way to determine this would be to disable ABS and see if the rear locks up, but it certainly FELT as though the rears were.

I put my brake pressure gauges on the system with the old MC and the new MC and it seems the pressures are very close to the same, but they come in quicker and at a higher point on the pedal. The 2002 MC is theoretically sending more volume to the rears, since the piston is the same diameter throughout on the 2002 MC (34mm), whereas the old MC is 40mm for the fronts and about 28.6mm for the rears, according to some measurements I found on another site (copied them into a Wordpad doc for future reference). So, the 2002 MC effectively decreases the piston diameter for the fronts and increases the diameter for the rears compared to the original MC.

Quote:

CrazyHoe

Now for the rear JB5....

I believe my rear brake cylinders are 1" in diameter, there is a 1 3/16 model for an increase of 41% in hydraulic pressure. Just a suggestion for when you'll re-do the rears...

Are the 1 3/16 models for JB5 or JB6's that you can use on the JB5? I just picked up a JB6 rearend complete drum to drum that I can swap in. What size cylinders does the JB6 ome with?

I do not want the rears locking up before the fronts so maybe we need more testing on the MC swap or am I misunderstanding?

TJ



Last edited by 98Blackss on Wed Jul 25, 2012 1:21 pm; edited 1 time in total

Well 34 vs 28.6 is 41% less pressure to the back (more volume). And 38% more pressure for the front.

98Blackss wrote:

Quote:

CrazyHoe

Now for the rear JB5....

I believe my rear brake cylinders are 1" in diameter, there is a 1 3/16 model for an increase of 41% in hydraulic pressure. Just a suggestion for when you'll re-do the rears...

Are the 1 3/16 models for JB5 or JB6's that you can use on the JB5? I just picked up a JB6 rearend complete drum to drum that I can swap in. What size cylinders does the JB6 ome with?

I do not want the rears locking up before the fronts so maybe we need more testing on the MC swap or am I misunderstanding?

TJ

I just looked up what's available for my truck and both came up. I believe they are physically the same on the outside.

JR96CK wrote:

Yes, the 2002 has discs in the rear.

I did some 60-0 panic stops both before and after the swap, and from what I can tell the rear was locking up. ABS was going crazy, so perhaps a better way to determine this would be to disable ABS and see if the rear locks up, but it certainly FELT as though the rears were.

I put my brake pressure gauges on the system with the old MC and the new MC and it seems the pressures are very close to the same, but they come in quicker and at a higher point on the pedal. The 2002 MC is theoretically sending more volume to the rears, since the piston is the same diameter throughout on the 2002 MC (34mm), whereas the old MC is 40mm for the fronts and about 28.6mm for the rears, according to some measurements I found on another site (copied them into a Wordpad doc for future reference). So, the 2002 MC effectively decreases the piston diameter for the fronts and increases the diameter for the rears compared to the original MC.

What tire size?

Crazy - Not sure I am following:
2002 MC is 34mm Front and Rear
1998 MC is 40MM Front and 28.6 for the Rear according to JR's post.

Front 34mm-40mm=-6 /34mm = 17% LESS
Rear 34mm-28.6mm=5.4 /34mm = 15% MORE

Am I totally off base?
TJ

CrazyHoe wrote:

What tire size?

Stock size:

P265/75/R16

Brand new Goodyear Silent Armors at all four corners for both before and after testing.

98Blackss wrote:

Crazy - Not sure I am following:
2002 MC is 34mm Front and Rear
1998 MC is 40MM Front and 28.6 for the Rear according to JR's post.

Front 34mm-40mm=-6 /34mm = 17% LESS
Rear 34mm-28.6mm=5.4 /34mm = 15% MORE

Am I totally off base?
TJ

Decreasing the size of the MC piston actually increases the force applied to the slave piston which is, in this case, the caliper piston, but decreases the fluid volume. Increasing the size of the MC piston decreases pressure on the slave piston (the wheel cylinder in this case), but in creases the volume.

In short:

Smaller master piston, larger slave piston = more force exerted on the slave piston but less volume of fluid moved.

Larger master piston, smaller slave piston = less force exerted on the slave piston but more voulme of fluid moved.

Thanks!

Its the volts/amps : pressure/volume thing...

TJ

98Blackss wrote:

Crazy - Not sure I am following:
2002 MC is 34mm Front and Rear
1998 MC is 40MM Front and 28.6 for the Rear according to JR's post.

Front 34mm-40mm=-6 /34mm = 17% LESS
Rear 34mm-28.6mm=5.4 /34mm = 15% MORE

Am I totally off base?
TJ

I calculated the % increase/decrease according to the area ratios.

It's not simply subtracting diameters.

JR is correct.


Now I wonder how much fluid is pushed in one stroke gmt800 vs gmt400.... but then again, if the pedal is firm and the wheels lock-up.. Who cares! loll



Last edited by CrazyHoe on Fri Aug 03, 2012 9:43 am; edited 1 time in total

JR96CK wrote:

CrazyHoe wrote:

What tire size?

Stock size:

P265/75/R16

Brand new Goodyear Silent Armors at all four corners for both before and after testing.

That's awesome! If you can trigger the abs with those big tires and stock brakes, you won't fear panic stops.

As for the different rear cylinders, I suggest measuring your braking distance from a set speed.

I recorded VSS data during the panic stops and thought I could look for perhaps a steeper deceleration curve to tell me if I'm stopping sooner, as I could think of no reliable (read: repeatable) way of measuring stopping distance directly with the equipment I have. However, I've only glanced at the data logs and have had no time to parse the data into anything usable or telling as of yet (but I will when I get a moment to do so, just out of curiosity). Given that my old AutoTap software is generally crappy (low frame rates), I don't know if I will be able to get enough resolution out of the logs to see anything worthwhile. We will see.

I've resigned to my seat of the pants feel for the brakes, ultimately, and since I'm doing a TON of other work to the truck over the next couple of weeks, I've had to move on and have not had a chance to collect my usual litany of data like I would normally do. The difference is obvious enough that I am quite certain the truck is stopping much more aggressively though. I'm thinking when I do the swap on my 96 pickup, I'll take the opportunity to get some more numbers and experiment some more.

Maybe check for the time it takes to go from xx mph to 0?

Actually tried that, but it won't let me graph time stamp data. Don't know why the software won't do that. Thought about tallying up the number of frames recorded during the stop, but that is not totally reliable, as frame rates may not be consistent from one recording to the next.

So it's the vacuum boosted MC as the hydro one is 37mm according to RockA.

Yep, vacuum boost JB5 brakes.

I got a new booster and mc ordered.

Any measurements needed?

JR96CK wrote:

Actually tried that, but it won't let me graph time stamp data. Don't know why the software won't do that. Thought about tallying up the number of frames recorded during the stop, but that is not totally reliable, as frame rates may not be consistent from one recording to the next.

What about logging a single PID, like rpm? I can't seem to get frame rates faster than 1/second with my software either. May get you close though.

Ok so the gmt800 MC piston is .160 below the surface so the piston to flange distance is : .647-.160= .487

But this gives little info on the actual gap because the pin tip is round and mates to a cone shape recess.

So with patience and a little set-up I calculated the gap between pin and the GMT800 piston to be about .030 +- .005

That's pretty good but I wonder if the pin could be longer by say.... 1/8.

How would the master cylinder behave?

How about on the 400 master to pushrod clearance? I replaced a master on a 97 yukon yesterday and did notice when I hit the pedal with no vaccum on the booster you could feel a good 1/2" free travel before you felt any kind of contact with anything.