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| | | | Joined: Jul 2003 Posts: 108 'Bolter | | 'Bolter Joined: Jul 2003 Posts: 108 | I have a 46 1.5 ton Chevy. I have freshly turned drums, new shoes, reworked the wheel cylinders and added a new Power Brake Booster (vac line is attached to intake manifold). Custom bracket, but stock location under the driver side floor. I have two residual valves @ 10 psi each and a proportioning valve.
The stock manual master cylinder piston was
1 3/8" and the new master cylinder is 1 1/8".
There is only 1/8" clearance between the rod and master cylinder it pushes on. I have bench bled the master and bled the brakes several times after a drive, and no more air comes out.
HOWEVER, here's my problem, I do not have any pedal, it goes almost all the way to the floor before it grabs. But when it does, it grabs nicely, not spongy. And sitting on a hill, I can just barely have my foot on it and it holds, and does not leak down.
So, all is right, except the pedal distance.
My question: Would a 1 3/8 piston on the master cylinder fix the travel distance? Or would that make it 'hard' to press again?? | | | | | Joined: Aug 2005 Posts: 19 'Bolter | | 'Bolter Joined: Aug 2005 Posts: 19 | Power brake master cylinders usually have a different pedal ratio than manual ones. The rod attaches further from the pivot to give more rod travel for the same given pedal travel.
I will post the correct ratio if I can find it.
You should use the same diameter or a larger piston than the manual version as well. This will ensure the proper amount of fluid flow to the wheel cylinders. | | | | | Joined: Mar 2004 Posts: 6,212 'Bolter | | 'Bolter Joined: Mar 2004 Posts: 6,212 | Your not getting enough fluid out to the wheel cylinders with your 1-1/8" small bore master cylinder (it's bottoming out just as the brakes grab). A larger bore 1-3/8" MC would push more fluid with less travel. For safety sake I think you need to find a larger MC. Mike  | | | | | Joined: Jul 2003 Posts: 108 'Bolter | | 'Bolter Joined: Jul 2003 Posts: 108 | Yea, that ratio would probably be helpful. I had to adjust it several times, but I was trying to get it back to 'stock'.
AND, the same diameter makes sense to me, but I had some 'guru' tell me it would make the pedal hard.
I said, Well, it's the same size as manual, and WITH a Power Brake Booster, how could it be 'harder' than manual.
Well after a lengthy disdaining, (he was an older gentleman) I went with the 1 1/8. | | | | | Joined: Mar 2004 Posts: 3,068 'Bolter | | 'Bolter Joined: Mar 2004 Posts: 3,068 | Sounds like you need to adjust your pedal to booster rod length. I did a simular upgrade to a 48 1.5 ton and it took quite a bit of adjusting to get everything dialed in. You will need to have some pedal travel after adjusting (meaning don't adjust to the point you only have an inch or so of freeplay) because all of your brake system is under the floor with the heat. Even worse on mine, it's a V-8 truck with an exhaust pipe running rather close to the master set-up. The heat will cause the fluid to expand and "take-up" some of your pedal freeplay.
Good luck,
Scott | | | | | Joined: Apr 2000 Posts: 247 Member | | Member Joined: Apr 2000 Posts: 247 | Make sure your brake shoes are adjusted out to where they brush the drums when the drum is turned. | | | | | Joined: Apr 2005 Posts: 1,107 'Bolter | | 'Bolter Joined: Apr 2005 Posts: 1,107 | i had the same problem with the pedal and i blamed it on the new MC. :confused: it ended up being that the shoes wernt adjusted out far enough. i adjusted them just till i heard them rub the drum. since then full strong pedal with no creep. shortly after i had corrected this syndrome, i had to lock them up one time unexpectedly  i had a full pedal on the first pump and those tires made a low pitch scream as they slid across the road  i think if i had to pump them twice i would have rear ended the woman in front of me that decided to turn at the last moment with no notice :mad: its sure is comforting knowing you got brakes when you need them  | | | | | Joined: Jul 2003 Posts: 108 'Bolter | | 'Bolter Joined: Jul 2003 Posts: 108 | Thanks guys, it seems so far, the best idea is to go with the M/C with the 1 3/8" bore.
I have already adjusted my brakes. I put the whole truck up on jack stands, and spun the tires until they would just barely grab and then backed of 1/4 turn.
Dang, I wish that was the problem. Also, before I 'buy' a new M/C, I will try to adjust the pedal booster rod length.
Boy, that would be an inexpensive fix.
It will be the weekend before I get to try these out, but will give an update as to what the final fix was. | | | | | Joined: Nov 2005 Posts: 6,100 'Bolter | | 'Bolter Joined: Nov 2005 Posts: 6,100 | Timmer if your master cylinder was designed for drum brakes it allready has residual pressure valves built into it so no need for extra valves. Adjust your brakes up hard so you cannot turn them this centers the shoes then back them off to a slight drag, but not more than seven notches. If all parts are kept adjusted your master cylinder will probably be okay, the only time you would definatly have a problem is when you let things get out of adjustment, more travel more fluid needed | | |
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