pushrod between the MC and the booster

[77SSII]

I was reading on another thread that mr. Mayben said that the pushrod out of the booster must be adjusted exactly, why?
Does the break peddle assembly not have enough stroke to make up for any slight mal-adjustment of that pushrod? I know if it's too long it will keep pressure on the mc piston and slightly drag the brakes right? If the pushrod is say 0.020" too short wont the peddle just need to be pushed a tiny bit farther before the pushrod pushes on the mc piston?
The reason I ask is that I have a totally new break system w/an older style TSM rear disk conversion (caliper mntg brackets doesn't aim the bleeder screw at 12 o'clock) and a new disk/disk prop. Valve (stock GM). I am using all -3 stainless tubing and fittings w/braided stainless lines (I fixed all the leaks). A new mc and a reman. Booster from napa w/the fitting. I do have some more bleeding to do and the truck stops well but I need to use two feet to really get it to stop hard. I'm only running 33s but it doesn't feel like I can lock up the brakes.
I have a mild schneider cam (grind: 256-2h) with my idle adjusted properly (my return spring wasn't set up correctly and I would have to pull up on the gas peddle to idle properly). I haven't done a vacuum check yet but I will soon.
I want to shave every possible foot off of my stopping distance.
Have I missed anything?

 

[Michael Mayben]

I was reading on another thread that mr. Mayben said that the pushrod out of the booster must be adjusted exactly, why?
Does the break peddle assembly not have enough stroke to make up for any slight mal-adjustment of that pushrod? I know if it's too long it will keep pressure on the mc piston and slightly drag the brakes right? If the pushrod is say 0.020" too short wont the peddle just need to be pushed a tiny bit farther before the pushrod pushes on the mc piston?
The reason I ask is that I have a totally new break system w/an older style TSM rear disk conversion (caliper mntg brackets doesn't aim the bleeder screw at 12 o'clock) and a new disk/disk prop. Valve (stock GM). I am using all -3 stainless tubing and fittings w/braided stainless lines (I fixed all the leaks). A new mc and a reman. Booster from napa w/the fitting. I do have some more bleeding to do and the truck stops well but I need to use two feet to really get it to stop hard. I'm only running 33s but it doesn't feel like I can lock up the brakes.
I have a mild schneider cam (grind: 256-2h) with my idle adjusted properly (my return spring wasn't set up correctly and I would have to pull up on the gas peddle to idle properly). I haven't done a vacuum check yet but I will soon.
I want to shave every possible foot off of my stopping distance.
Have I missed anything?

The critical part about adjusting the booster pushrod is to make certain it it isn't "too long". And even 0.020" "too long" is too much! As the brake system heats and the underhood temp increases in a normal fashion, then the brake fluid expands slightly, but enough to actually lock the brakes up until ya pop the hood and let the system cool down and everything returns to normal until heat is applied again.

If the pushrod is too "short" in adjustment, then any excess free play between the booster and the mc will be greatly magnified and the booster can't supply appropriate travel to the mc.

The freeplay in the brake pedal-to-booster is adjusted after the booster/mc is set up correctly. I know it seems like this "tolerance" thing regarding the two pushrods should be kinda secondary, but it's not, the actual line pressure generated by the booster/mc combo is greatly affected by the two freeplay adjustments and magnified in pedal feel and greatly eats up pedal travel.

Are you certain that the master cylinder is correct for the disc brake application? Is the booster the proper one for disc brake also (dual diaphragm, the housing is approximately 9" thick, not the "pancake" style used with drum brakes).

We gotta know what the manifold vacuum level is at idle also, it takes a minimum of a continuous 15"hg to properly actuate a vacuum brake booster. Any of these engines in "average" tune will make 17"hg minimum vacuum when the throttle is chopped. The difference of only 2"hg in vacuum is huge when dealing with brake boosters.

Your description tells me that ya got a booster issue...either it's not operating correctly or the manifold vacuum supply is too "low".

Also...if the proportioning valve you used is not biased for a vehicle with the front/rear weight distribution of the Scout II, then the balance in the system is greatly altered, that is why swapping that kinda stuff around when adding a brake system conversion like this is not a good solution. The best solution is plumbing in an adjustable valve in the rear circuit only and then setting it up with some test drives and locking down and not ever changing it again.

Just two weeks ago, I installed a new centric mc (in actuality it's a raybestos unit inside the box!) on our recently acquired t'all. I did a rudimentary/quickee bench comparison of the mounting flange-to mc piston recess dimension and it was within 0.035" of the same (short) so I slammed it on after bench-bleeding to see what would happen in a typical service replacement just like our customers do weekly. Result...a half inch of excessive pedal travel, just as if the pedal pushrod was too short. So...I "lengthened" the booster pushrod 2 turns (which was about 0.040", those are very fine thread pitch!) and that made it perfect! At the same time, I found the nut that retains the brake pedal pushrod pivot to the pedal to be hanging on by only one thread which allowed a huge amount of slack in the system (I'm still getting all the po bugs worked out!). Once the entire system was verified as correct, the pedal feel and oem stopability mysteriously returned to normal!

 

[77SSII]

Thanks michael,
I definitely have to tune the carb and hopefully I can do this on Friday.
I have bent stainless tubing going from the mc to the prop. Valve and I'm going to replace them w/braided stainless. With those on there, there should be enough slack to take the mc on and off w/out the hassle. I have banjo bolts on the mc now as there isn't enough clearance btwn the mc and coolant overflow jug to bend and flare the tube!!!!! When I tighten the banjos for the crush washers, it is a huge pain to get the bent tube lined up properly for a good seal so here come the braided lines.
So I'll bleed everything again and hopefully w/the tuned carb I'll see some improvement.
I spoke to skip over at TSM and he said he's sold a ton of these for scouts and the stock mc should be sufficient even though Jeff sell a different mc. This "stock" mc comes from a Scout guy and seems to be working.
I bought a reman. One years ago at a local parts store and the f-in thing had a residual pressure valve for the rear brakes and after some time the truck would stop by itself and refuse to move!!!! Pop the hood and wait a 1/2 hour then you could drive another 5 miles or so 'til it happened again!
The calipers from the same store have a Ford logo on them! They just cut them to fit into the Scout brackets!
Man, sometimes I feel like I should have bought a mustang

 

[jmalloy]

Old post, I know -

just wanted to add a "thank you, mr. Mayben!" my front brakes were grabbing when the Scout had been running for any length of time. Adjusting the pushrod as described above (1/2 turn) quickly and easily took care of the problem.