Why a Dual Reservoir Master Cylinder isn't necessarily safer (and what you can do about it).

When considering a master cylinder for my disc brake conversion, I wanted a dual-reservoir set up for two reasons: 1. It separates front and rear brakes, so I can apply power boost to the front discs where I need it most, and 2. It provides a "safety feature" that a leak in front or rear circuit will not totally disable the brakes; the other circuit will still work.

When I installed the master cylinder, I didn't change the attachment point for the push rod on the brake pedal arm. I did't really understand the reason I needed to, even though I had read on Gil Bumgarner's blog (step 27) that it was necessary.  Here's a photo from his blog, with my notes.

So, why is it necessary to drill a new hole and reposition the brake rod bolt when installing a dual-reservoir master cylinder?  After all, it's really difficult to get to that bolt after the steering column is installed... and I mean REALLY difficult!

Well, here is the reason.  The stock 55-57 T-bird brake master cylinder has slightly less than 1" stroke.  A typical dual-reservoir master cylinder has about 1 1/16" stroke.  Re-drilling the pedal arm seems like a lot of work for a little over 1/16"... so I decided not to do it and see what happens.  For 3 years I have been driving this car, and "nothing bad" has happened (yet).  I have been playing with the brakes, changing the power booster and the plumbing, and had some exciting moments on the road.  The pedal felt low at times, but not really "unsafe".   Here's what convinced me to actually follow Gill Bumgarner's instructions:

I was driving on surface streets after a long drive and I noticed the brake pedal was lower than usual. In fact, I could push it all the way to the floor, and still not get full brakes!  By pumping the pedal, I could get the car to stop.  I was near a Pep Boys, so I pulled in and checked the reservoir... sure enough, the front reservoir was empty.  I purchased some DOT 4, filled it up, got my brakes back, and was on my way.  Running low on fluid is my fault, it is not unusual for fluid to drop (without a leak in the system) after major work like replacing a power unit.  I simply forgot to check before I left.

Still, it got me thinking... reason #2 for dual reservoir brakes should not allow the pedal to go to the floor with no brakes at all!  I recall a T-bird club member who got a dual-reservoir master cylinder on his drum brake car who had a similar "brake pedal to the floor" experience, so this is probably not unique... but why does it happen?

Back to that pesky 1/16" of stroke... it turns out that the last 1/16" is the most important!

There are two pistons in series in a dual reservoir MC.  When brakes are applied during a leak (or empty reservoir), one of the pistons immediately goes all the way to the end of it's travel.  The remaining piston now has only half the stroke to develop effective brake pressure.  The very last part of the piston stroke may well be necessary to develop this pressure!

So, how to provide more stroke to the master cylinder?  There are two options...

1. Do as Gil Bumgarner suggests, and drill second 3/8" hole in the brake pedal lever (part 2455 in the above diagram) about 1/2" below the current hole, and reposition the brake push rod bolt (part 2462) to this hole.  As mentioned above, this is a real pain to get to.
2. Remove, trim, and replace the brake bumper stop (part 7583) so that the brake pedal is higher when resting.   If this isn't done at the same time as 1. above, the push rod will need to be longer, and adjusted/replaced.  So, you will STILL need to go through the pain of removing the brake push rod bolt!

Which did I do?  Both of them.  Took a couple of hours, and I had to fabricate some tools to remove the brake push rod bolt.  Here's a pic under the dash:

Very tight in there!

Brake Push Rod & Bolt

Nut on the other side of the brake push rod bolt.

It is very, very tight under there.  Luckily, I had a crow's foot wrench set that I could modify to reach the brake push rod bolt (2462) and it's locknut (34395-S).  Removing it required tenacity, limberness, and patience.  I was then able to push out the rubber bumper stop, cut off about 3/8" of the rubber, and replace it.

Then, I carefully marked where the new hole needed to be on the brake pedal lever, making sure the pedal was all the way up (against the modified bumper stop) and the push rod was fully engaged in the master cylinder.  I used a silver sharpie...

I lined the floor with butcher paper to catch the metal shavings.

To drill the hole, I had to use a close-quarters drill ($30 at Harbor Freight):

After carefully drilling a pilot hole, I drilled the required 3/8 hole.

The brake push rod bolt has an eccentric to it, allowing up to 1/16" adjustment.  This is really just for taking most of the slack out of the brake pedal, but it is very convenient.

The result?  Well, I haven't deliberately induced a brake failure (yet), but I can check the emergency operation of the dual reservoir MC the next time I bleed the brakes.  For now, I'm satisfied that the brake pedal seems to rest about an inch higher, the pedal is about 2" off the floor under heavy braking, and the brakes feel quite firm and solid.  I declare victory!


That said, here are some glam shots of recent additions to the engine compartment... just for fun!

The decal on the heater duct makes it look more complete.  Plus, it was cheap.

The windshield washer bag fills a void space on the driver's side of the engine compartment.  And it's colorful.

Till next time....