Question about brake booster pushrod and Master Cylinder pre-load - almost done!

[Corellian Corvette]

I've converted my brake system over from Drums to Discs using an original bendix brake booster and parts from the MP brakes disc brake kit.

The Master Cylinder I have is from the MP brakes kit. It looks very similar to this, with a 1 1/8 piston bore:


The system appears to stop pretty well, but the pedal goes WAY to the floor before the brakes engage, which I don't like.

Since there is no pushrod length adjustment inside the car, I assumed that the pushrod on the booster side must be set incorrectly. I pulled the MC off, and the pushrod "head" was backed all the way down.

What I also noticed is that the Master Cylinder piston is not flush with the back of the casing, it actually sticks out maybe 1/8 in.

The service manual calls for .920 length of the pushrod from the face of the booster and mine was WAY off. I adjusted the pushrod to the factory specs.

When I put the M/C back on, it caused some "preload" on the piston, pushing it back in (to about flush) - so let's say the piston is now depressed about 1/8in of 'preload'. Looking inside the M/C, there are two little sight holes where you can see the piston in each of the bowls. Before, the piston was at the center of the sight. Now, the piston is moved forward to the front of the sight.

I got back in the car and hit the brakes and they are FIRM. I have not road-tested it yet, but the feel is perfect.

Is this going to be OK? Is it correct to have some pre-load on the M/C piston? Or am I causing some other problem?

Thanks!

[Just 6T9 CHGR]

That sounds about right....I followed these directions...

[John_Kunkel]

If there is any "preload" on the piston the plunger is adjusted too far out, you want a few thousandths gap.

Once the piston seal is has passed the compensating port, the system is effectively sealed and no fluid can return to the reservoir, this is why the pedal feels so firm. If you drive it like that the brakes will likely lock up.

[Corellian Corvette]

Well I used the 2nd method - rod is adjusted so that fluid "squirts" at about 1/2 in pedal travel.

Problem still is that the pedal goes way down to the floor before stopping the car.

The brakes work - I mean the car stops no problem; it's just that there is no 'progression' to the brakes - feels like nothing is happening then fairly quickly - brakes are applied.

I wish there was an under-dash adjustment (like on GM brakes) so I could lenthen the pedal to get more travel. That might give a more linear feeling.

Otherwise i don't know what else to do. The pedal isn't 'squishy' so it doesn't feel like air in the lines.

How can I tell if the bore size on my MC is big enough?

The car has 4 wheel discs, FYI.

[tommymac]

Say? what MC is that? and can I bolt that up to a stock Booster?

[Corellian Corvette]

Are you using 4 wheel discs or discs/drums?

Check out the MP brakes.com website if you are using discs/discs.

Also, if you just have front discs, you can go to NAPA and get the correct disc/drum master cylinder from them. Many have purchased those and are very happy with them.

[Just 6T9 CHGR]

I have used both & both work well....I believe the one that MP & SSBC uses is the A body unit....

1st pic is the A body one

2nd pic is the NAPA correct Bendix replacement

[Corellian Corvette]

Hey NOT Just 6T9 CHGR! Glad you posted I have another question for you!!

Where did you get the mouting pieces/clips that hold your Vacuum hoses to the inner fender? They don't come with any kits I see, and I don't know what they are called so I can't locate them.

Your engine compartment detailing is AMAZING!

[LaOtto70Charger]

For the correct NAPA MC which vehicle is it for?  73 duster with disc brakes?

[Just 6T9 CHGR]

For the correct NAPA MC which vehicle is it for?  73 duster with disc brakes?

See here for the correct one for 68-70
http://www.dodgecharger.com/forum/index.php/topic,30312.0.html

[LaOtto70Charger]

Thank you for the link.