New Master Cylinder on Original Booster Question

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kastlerock

New member
I had IH north build a front brake conversion for my sii. I have just recently got the brakes working (somewhat). I replaced my master cylinder with a new raybestos model (purchased from IH north as well) and attached it to my original booster. I have also bled the brakes. My problem is this:

1. When I press the brake it feels as if the booster moves the plunger/rod but it feels like it doesn't hit the master cylinder until about half way through my pump.

2. This causes the brake pedal to be extremely low when it finally starts braking; with little room between the pedal and the floor.

My question is whether the booster rod needs to be replaced with a longer one or if I am doing something else wrong. Any help would be appreciated.

Thank you,

david
 
Same (but opposite effect) situation is discussed here:

http://www.forums.IHPartsAmerica.com/brake-tech/3582-brake-bleeding-round-3-a.html

Your current pushrod between the mc and the booster needs to be set correctly for length (it's adjustable). It' very tedious to get it exactly right!

The service manual outlines the procedure with some actual dimensional data, but that data is correct only for the oem mc and the oem booster. Any aftermarket replacement parts are gonna vary a bit and there in lies the problem, ya just gotta do some rudimentary calculations and measurement to get the mc pushrod length just right, and that can only be done with the exact components you are trying to put together, not some stuff out of a service manual!.
 
Thanks for the response. We have made the adjustments and it seems to have fixed that problem but I am still having some issues. We took the rig out for its first test drive last night around the block and found that it does stop but not well. We got up to maybe 25mph and hit the brakes and, though it stopped, it didn't stop well or fast. I would think it would have tried to skid with the amount of pressure we applied. The pedal feels as if it is bottoming out; not on the floor but in its arc. The Scout is a 73' that had full drum brakes. I am using the original booster and wonder if I should replace it with a more contemporary one; maybe a 74' or up that had started to use disc brakes in the front. Any thoughts? Also, the pushrod assembly in the booster seems to be a bit damaged around the gasket and I wonder if that would cause a vacuum leak or the symptoms I am seeing.
 
Several ideas here, but I don't know what all else you have changed out so I'll start kinda from scratch.

I gather that you obtained new disc conversion parts from Jeff at the shop along with a "new" disc brake master cylinder.

The drum brake vacuum booster is a single diaphragm (what we call the pancake model) and is barely adequate for the oem drum brakes. The oem disc brake booster is the dual diaphragm unit (about twice as thick in the body area). The pancake booster will work...but does not provide enuff boost/over-ride to really clamp the new calipers on the front without a very strong push from yore laig!

The drum brake system used only a "combination" valve which is used to operate the brake failure warning light system, that is all it does.

The disc brake system used a "combination/proportioning " valve which proportionally reduces line pressure to the rear brakes when the front brakes are applied. So either that part or a workaround is mandatory for a disc/drum combo brake system like you now have. The Scout II service manual explains all this stuff in detail and shows all the variations I'm describing.

If you do not currently have a functional proportioning valve, then the workaround for that is to plumb the front disc brake system direct off the rear reservoir of the disc brake mc using a union in the line if needed.

Then install a manually-adjustable proportioning valve in the line to the rear brakes only! Such a valve is readily obtainable, ihon has 'em on the shelf. Then play with the setting of that valve to obtain correct braking performance and lock it down, you will not change that setting again.

You will not have any method of illuminating the "brake warning light" by doing the workaround, but who needs nannystate shit like that? I don't need nhsta or cpsc telling me my brakes left the scene!

If the pushrod between the booster and the mc piston is burred, then take it out and "polish". That is pretty common and tells me someone used visegrips or channelock pliers on it to adjust.

Rig a vacuum gauge on the intake manifold and once the engine is warmed up, note the reading. It must be steady (not wavering/wiggling) and needs to be a minimum of 16"hg at your base elevation in chico. Up at paradise on the hill it will be about 2"hg less due to increased elevation!

Then while watching the vacuum gauge, pump the brakes and note the needle movement. It will "glitch" once and return to steady, that is normal. If it's holding steady vacuum when the vacuum booster operates, and the vacuum level remains a minimum of 16"hg, then ya don't have a vacuum leak. Also replace that check valve and grommet that is plugged into the front of the booster, those are about $3.00 and are a "maintenance" item.

If ya can't make 16"hg vacuum, then ya got an engine tuning/performance issue to contend with!
 
Thanks for the reply. I will scrutinize your recommendations more closely later today. To give you an idea of my setup:

1. 73' sii - originally had drum brakes all around
2. Purchased custom front axle with GM disc brake conversion from IH north
3. New mc from IH north
4. Built all new custom brake lines
5. Using a wilwood proportioning valve
6. Using a hurst line-lock
7. Using original brake booster - maybe I should replace this
(come with pushrod assembly?)
8. Purchased and installed deluxe drum brake kit for rear from IH north
9. Did not install parking brake assembly
 
kastlerock said:
thanks for the reply. I will scrutinize your recommendations more closely later today. To give you an idea of my setup:

1. 73' sii - originally had drum brakes all around
2. Purchased custom front axle with GM disc brake conversion from IH north
3. New mc from IH north
4. Built all new custom brake lines
5. Using a wilwood proportioning valve
6. Using a hurst line-lock
7. Using original brake booster - maybe I should replace this
(come with pushrod assembly?)
8. Purchased and installed deluxe drum brake kit for rear from IH north
9. Did not install parking brake assembly
Click to expand...

Excellent!

Ya got all the stuff needed! That's kinda what I thought but I had to ask!

The booster May be the key to this deal! And then...dialing in the adjustable p-valve once everything is operating normally.

So it appears the only unknown here is manifold vacuum level/analysis.

Also as an aside...the use of significantly greater rolling diameter tires over the oem size greatly affects brake performance/pedal feel.

I assume you have the line lock installed in the rear brake system for use as a parking brake? That should not affect anything, it's a simple on/off valve.
 
I vote either push rod is the incorrect length or you need to bleed it again or your booster is not up to snuff.

With the system you described you should have the rear locking up everytime you hit the brakes - until you get the proportioning valve dialed in -- once you have that dialed in it should skid front then rear a split second later. I had 35" tires on a very similar setup and had great brakes and good pedal feel.
 
I am going to assume it is the booster as we have bled the brakes twice and adjusted the pushrod multiple times to get it just right. I too am running 35" tires so that could be an issue with the booster as stated above. I have placed an order for the newer type of booster so hopefully we'll have a better idea by next week. I was hoping I could put that 383 stroker to the test but that isn't going to happen until I can stop the darn thing!
 
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