test

Dump question but did you adjust the rears up until they scraped a little?

I did indeed...

I am befuddled.

Of note, through all of this I've replaced both rear wheel cylinders (one defective, and one because I needed a later style one to go with my AL master cylinder (bran new for an 89 Diplomat) and the RF caliper after I broke the bleeder screw off.

I've pumped probably 2 gallons of fluid through the system when bleeding and after the 2nd to the last bleed, I got some nasty fluid out. On the last bleed, the fluid was foggy, but not as bad as the previous bleed.

Could that caliper be hanging up some how?

put a block of wood under the pedal to limit its travel to the normal height (anytime, when checking or bleeding). clarify about the pedal stopping halfway down (which it would do if the brakes were functioning normally), more details please

Gotcha on the block of wood.

The pedal is stopping halfway down (which indicates normal operation) however no matter how hard I stand on the pedal, no lockup and longer stopping distance. The initial bite is as it should be. The car stops just fine in traffic situations, but when jumping on the pedal at ~30 MPH, nothing locks up and the car comes to a stop with no drama, albeit in much more distance than it should.

Pedal doesn't not pump up

Pedal "bottoms out"(better description than "stops halfway down") in all braking situations

Adj prop valve fully open (full flow to rear)

I've got to think there is more air in there, but I've bled bled bled and pumping the pedal doesn't change the system behavior.

this is power discs/drums right? (1) recheck the round pushrod nub clearance (2) take the line off of the MC front port (for the rear brakes) & cap the MC with the brass inverted flare male plug fitting & drive it & see how it acts with us isolating the front discs

Will do.

I also had one of the MC-Prop valve coiled lines with one coil above the MC. Rectified that and will bleed, test and recheck the pushrod clearance before isolating the front.

Might not get the opportunity until Monday though.

Allright, so here is the latest.

1st, I have NOT yet isolated the front brake channel and tested.

What I have done is re-bleed the brakes (using a 2.4 under the pedal) after re-routing both MC-prop valve lines such that all of the coils are below the level of the MC mounting. I also visually verified brake operation at all four corners.

The result is a HARD pedal that feels as it should, and normal braking is as it should be. That said, no matter how hard I stand on it at any speed, not even the rears lock up and it gently slows to a stop after a much longer distance than it should. The pedal does not pump up, and braking begins immediately upon pushing the pedal.

Could this symptom be caused by an MC mismatch? I replaced the stock replacement (Big A Auto no less!) MC with a KH unit from Rock Auto listed for an 89 Diplomat. It is listed as having the desired bore. Several aftermarket brake manufacturers list "Master cylinder bore size too small for the system volume requirements." in their troubleshooting guide for "Poor Brake Pedal" but the symptom is no more specific than that.

I had that real drawn out stopping distance once & it was a partly crimped front line (before the splitter so both fronts affected) on the stock car but people have reported that too large of MC bore dia will give an inodinately hard pedal to the point of literally having their a$$ up in the air stomping on the pedal to generate any force but that ain't been my exp with the 1&1/8 MC I used on my 65 dart (& I ain't that strong). what Bore is yours? I gotta lean toward a partial blockage of some sort & I would go ahead & isolate the halves again for a start

I'll double check the lines and isolate, I did re-flare 4 lines and the 3/32 jaw in the bar was a bit worn, so caused some frustration.

On the MC, it is a 1 1/32" (got sucked in by the price, $17.59). One wouldn't think 3/32" would make that big a difference, and wouldn't the line pressure be higher with the smaller bore? Could it be a matter of fluid volume? Could the stroke be less on the 1/32" MC?

Rear brake performance can be affected by wheel cylinder size as well.

Larger bore rear wheel cylinders put more force on the shoes than smaller bore.

You all may be focusing on the wrong end of the braking system.

The rear cylinders are new stock replacements. I considered doing the Ehrenberg small wheel cylinder trick, but as my Prop valve was hosed, I went with a Wilwood integrated distribution/prop valve, so stayed with stock replacements for the wheel cylinders.

At this point, it doesn't seem to be getting enough pressure to the front or rear. I am unable to get even the rears to lock with the prop valve wide open.

Stock for what though?

Cause you say this



And it's confusing.

Heck, that confused me on the re-read.

>> In the mid-1970s, Chrysler eliminated the rear brake residual pressure valve from the master cylinder. Its function was to prevent any air from sneaking into the system via the rear wheel cylinder cups. In its place, Chrysler substituted "expanders" in the rear wheel cylinders. (Expanders are dish-shaped steel inserts that exert outward pressure on the cups.) If you should decide to use a master cylinder from a 1975-up car, be sure you have the expanders in your rear wheel cylinders. For at least 20 years they have been universal in all replacement wheel cylinders and rebuild kits.

Lest I introduce more confusion, the car is a factory power brake, disk brake car. I'm not converting to disks per this article or anything like that.


The replacements are O'Riley house brand new cylinders for my car (73 Barracuda).

So, you have a mostly stock 73 Cuda disc brake car with a late model M/C?

I did that when I converted my 65 Cuda to discs, Mushy pedal no matter what so I just went with a 4 bolt disc M/C.

Yes, stock but for the MC and aftermarket combo valve.

Lines inspected, no kinks or reduced line diameter anywhere where I monkeyed with them. I did not chase all the lines to the end of their route however.

Rear channel isolated, noticeably reduced braking performance, but still unable to lock the fronts. De-isolated the rear channel, bled and back to the former situation.

The 3/32" smaller than stock bore diameter is all I can think of that *may* cause this symptom at this point. This will teach me to cheap out. Anyone need a new 1 1/32 MC? I may have one available

Any other thoughts on troubleshooting avenues guys, before I part with more cash?

PS, I just noticed I failed to cite the Ehrenberg Disk-O-Tech article a couple of posts back. I poached that text right from the MA website.

I still think not enough psi is reaching the corners. I also dont think the 1&1/32 bore is a problem. a 2000 psi gauge hooked to the corners would ans that (I have one & it was cheap). you know what I would do, just me I would run some parts house prefabbed 3/16 line from the MC port next to the firewall to a brass inverted flare "T" then run both outlets to the front discs (with the other MC port capped). with it bled out if it wont lock em it'd have to be the calipers (one or both) or the MC. those lines are pretty reasonable as are the fittings & you have been comfortable with the fabbing you have done so far in diagnosing this

way back when i had a older (70's) winnabago motorhome come into my shop with back brakes completely burnt up,even cracked one drum,someone had replaced the master cyl about 300 miles back down the road,i replaced everything on the rear brakes,took it for a drive and it wouldnt stop well just slow down no matter how hard you pushed on the petal,after some head scratching i decided it had the wrong (to small) master on it)put the correct master on it for a 400 series truck and that fixed it,however the problem started when the owner left the other shop he forgot to release the parking brake,and drove it that way for around 300 miles without knowing it
hope this helps some
a true story!!

So a buddy/co-worker of mine moonlights at an Advance Parts that is attached to a warehouse. He arrived this AM with a 1 1/8" MC listed for an 89 D150 (a CarQuest one, no less and not their house brand, fruit of a merger). $27 and he'll take it back for me if it doesn't fix my problem.

Looks like I'm going Air Force Method for this step (throw parts at it until it works).

If this works, YAY! If not, I'll pick up a pressure gauge and continue in a diagnostic fashion.

I'll report back, and thanks for you input thus far everyone.

Chris