This is on my '72 W200. It was drum/drum, and now it's disc/disc. 2.94" diameter calipers up front, and the Caddy eldorado 2.5" calipers w/ebrake in back. 1.25" diameter disc/disc master cylinder from a '98 dodge. Stock vacuum booster.

The brakes work but they aren't great. I got the SSBC brake pressure kit a while back, and found I only had 800psi at the front calipers with the engine running. Today I tested the master cylinder itself, and I'm getting ~1000psi there. A little better, but still not where it should be. I plugged both ports in the MC and had a rock hard pedal.

The pictured part is the only thing left in the stock brake system. Do I still need it? My FSM refers to a metering valve in disc/drum setups, but doesn't talk about this part for drum/drum applications I'd like to keep the warning light if possible.

Later today I'll pressure bleed everything and see if I can get 1000psi at the calipers, assuming this part doesn't need to be gutted or something else. Then I may look into going with a hydroboost set up to increase the line pressure

It's a distribution block, in a drum/drum setup there is really nothing in there other than the warning light shuttle that will turn your brake warning light on if you lose either the front or the rear brake pressure. I suppose it could be pushed over a bit and partially blocking the port. I have taken them apart and cleaned then out and reused them.

Ok, cool. That's what I thought, but wanted to make sure.

I just pressure bled the front calipers, and checking the pressure on the front MC port (that goes to the rear calipers), I had ~900-1000psi with the engine running. Had the same PSI at the front calipers with the front MC port plugged.

However, when I really press on the pedal (engine running), the pedal comes within ~.75" of bottoming out. It seems like it should have more travel left than that, especially with just the front calipers hooked up?

Went through a quart of fluid this morning bleeding* the rear calipers and got a lot of air out. But with the engine running, I can still bottom out the pedal

*A while back I built a device that clamps over the master cylinder inlet, and lets me pump in compressed air to push the fluid through the system. I have the regulator set to ~10psi, and just give it 1 second shots of air while I watch the fluid come out. From what I've read, this method should work great, right?


I also checked the booster pushrod adjustment this morning. When I first unbolted the master cylinder, I got a hiss from the booster. Does this mean my pushrod was too long before?

Stupid thing is driving me crazy

Gravity bled the rears again, and still, if I reef on the pedal with the engine running, I can get it to bottom out in the master cylinder. Usually if you have air in the system, you can pump the brakes and the pedal will come up. My pedal does not come up if I pump it, which leads me to think I don't have any air.

My front & rear calipers are the same size as a stock Caddy eldorado which came with a smaller 1 1/8" master cylinder, so I'd think my 1.25" MC should move plenty of fluid.

Did find some guys online recommending a 37mm (1.45") bore master cylinder from an '02 Silverado

Have you driven it?
I recall some of the disc brake stuff having a lot of caliper/caliper bracket stuff, or rotor, having enough flex to allow the pedal to travel deep when at a stop, but would work normally during driving...this might indicate a misalignment of the centering of the caliper on the rotor or in the mount? Caliper not moving?
Just a thought...

Did you adjust the rear pads tight against the rotors before bleeding?

You must "ratchet" the parking brake lever to do this.

Both pads must be tight against the rotors. 0.08" clearance is too much.

Don't install the parking brake cables until the system is working correctly.

Un-hook the parking brake cables.

Pump the parking brake levers until the pads are tight against the rotors. 0.08" clearance is too much.

Both parking brake levers must be tight, and they should not "free-wheel"

Unbolt the rear caliper and position it over the rotor so the bleeder valve is level, not pointing down. Tap the caliper as you bleed it to dislodge air bubbles.

Hmm, I wonder if I just need another caliper then. I didn't take the cable off, but I did pump the arm quite a few times, and it would just suck the pad back each time. Both calipers were rebuilt ones from a local place.

I also had both calipers unbolted so I could move them around and tap on them while bleeding.

Sounds like typical rebuilt calipers.

I recommend new ones.

http://www.ebay.com/itm/320939288492?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649

the tms/tsm kit doesn't line up the rear calipers properly to bleed them on the car. not sure if that is what you are using, but you have to pull them off the car to get them to line up the bleeders to get the air out using the eldo calipers.

Times fun when you're having flies err, wait a minute, that's not how it goes...

Anywho, finally got around to replacing the bad caliper with another rebuilt one. No more gap between the pads and rotor, and the pedal feels better

But, the pedal still has quite a bit of throw. I saw something about that being possible if the pushrod is too short. So, while looking for the poof of fluid inside the master cylinder reservoir when the pedal is pushed, I instead noticed bubbles coming from inside the cylinder, up into the reservoir

Dumb question I know, but is there any way for air to enter the master? If I get a few tiny bubbles each time the pedal is pressed, it would either have to run out of air eventually, or air has to be being introduced somewhere?

I would have thought any air would have worked itself out after a couple years?

Anyways, planning to re-bleed the master, and everything else, sometime soon.

Time for the 2016 update to this project:

A while back I talked with Dr Diff over email and learned that the drum booster has less assist than the disc booster (single/dual diaphragm). So I swapped that out with a disc booster, as well as a matching 1-1/8" master cylinder. This brought the line pressure up a bit, but I still had excessive pedal throw.

Then I checked the rotor runout. Fronts had 15-18 thou (!), and the rears had 5ish, causing pad knock-back and a low pedal. Last week I had a local place use their on-vehicle brake lathe to clean that up. Now I measured runout of 0.001"-0.0015".

That should have brought the pedal up, but it didn't. Found that the calipers were retracting enough to leave a 0.016" gap between the pad and rotor. A few years of knock back and only 500-1000 miles per year must have stuck the piston to the seal. Spirited braking didn't fix this.

Rebuilt calipers took care of that problem, and now both sides are against the rotors.

But the pedal still has excessive throw :huh:

Fronts were pressure bled, and I took the calipers off and spun them around while gravity bleeding the last bit to make sure there was no air hiding somewhere.

If I pinch the soft line going to the front axle the pedal is high and firm, which also shows the low pedal is not caused by the rear discs.

So, about the only thing left is bulging soft lines at each caliper. Eventually my next move will be to get some braided lines to minimize any bulging.

My eventual goal was to go a step smaller on the master cylinder to raise the line pressure, but that will drop the pedal even more than it already is, and I can't have that.

Perhaps the final update:

I went ahead and installed some braided stainless lines on the front axle (the center line to the axle, and the two outers to the calipers).

With the engine running and my wife pressing the pedal as hard as she can, I'm getting 1200psi at the front calipers.

The pedal and braking effort itself feels better.

As for the pedal throw, if I pump it with the engine off until all the vacuum reserve is gone, I have 1-1.25" of free play in the pedal. The pushrod is adjusted to a gap of about 0.020". I'm not exactly sure where that much play is coming from.

Subtracting all that free play makes the pedal throw not near as bad.

So, it's finally pretty decent.