I have a problem that my break pedal goes soft after five minutes of driving. A quick dobbel pump of the pedal and it's hard and feels right again. Suspected trapped air in the brake system but after traditional bleeding, bleeding with vacuum and reverse bleeding (pushing fluid from calipers) I find it hard to believe that there is still air trapped in the system.

Current brake system is based on SSBC A-156 four piston calipers with a 11.25" rotor, OEM drum drum distribution block, SSBC A0707-1 prop valve to adjust pressure to the rear circuit, Doctor Diff 15/16" master cylinder (manual brakes) and Doctor Diff 10,7" rear disc brake kit based on Ford Mustang cobra OEM calipers.

I had the same problem last summer with the same brake set up except from 10" drums in the rear and SSBC 1.032" master cylinder up front.

Starting to lean towards that the problem could be related brake pad knock back so I checked for play at the wheel bearings but they nice and tight.

Anyone have any Idea that could cause this problem?

Clamp down either the front or rear hose(s) to pinpoint the problem.
Any residual pressure valves present in the system?

Rotor run-out can also knock the pads back

.. but wouldn't you feel the pulsation pretty clearly in your feet, via the pedal? ..and he checked the runout..

not that I have an answer.. unusual for a disc brake setup... if this is behaving different than last year when you had drums, I'd focus on the new disc setup you changed.

- Art

Pad knockback due to bearing compliance when cornering isn't all that uncommon for aftermarket setups, and will not provide you with a pulsation.

I had this sort of problem and it was the rear wheel cylinders, that I had replaced.
The part number was correct but it was really a wheel cylinder for the earlier Drums.
here is a clip from Mopar Actions Rick Ehrenberg's Disc o tech.

>> 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.

Opps, I jumped ahead of your question. I see now you have rear discs. I don't have an answer.

possibly install an aftermarket 2# check valve like speedway motors and other outfits sell ?

67sat, he checked bearing play, not run-out. Also, run-out by itself doesn't cause pulsating, that's thickness variation which can be caused by run-out.

At least we can pretty much rule out the rear brakes as the cause. But...

Is the problem more noticeable with the difference in master cylinder bores? Thinking the larger bore would have covered it up to some extent.

any resolution on this?

I would go through and tighten all the brake lines with line wrenches to make sure no air is coming in. Might be in an area where you won't see a little drip in a short drive. Mike

Using a dial indicator check the end play on the rear wheel bearings.
Set it to 0.0050 inches

Using a beam style torque wrench set the front wheel bearing pre-load to 125 inch pounds whilst spinning the rotor-after seating the bearings of course. INCH pounds.

That's a bit tighter than stock, stock spec is .008*.018"



That's half the procedure and the spec is off. Per the 72 Plymouth FSM, other years similar.

Adjustment:
(1) Tighten wheel bearing adjusting nut to 90
inch-pounds while rotating wheel.
(2) Position nut lock (Fig. 10) on nut with one
pair of slots in line with cotter pin hole.
(3) Back off adjusting nut lock assembly one slot
and install cotter pin. The resulting adjustment
should be zero (no preload) to .003 inch end
(4) Clean the grease cap, coat inside with wheel
( 5 ) Install wheel covers and lower vehicle to floor.

If you tighten to 125 in/lbs and leave it there, you'll be back doing new bearings.

Take the fsm and ignore it as 22+ years of laps says
125 INCH pounds


End play on rear bearings 0.0050

Last car I built had 4 wheels discs with no issues...
I forgot you guys are smarter than me

You leave that at 125 inch pounds and you'll be putting in bearings sooner rather than later

After further investigation of my problem I'm 99% sure my problem is caused by deflection that knocks that pads off the rotors. I measured rotor run out with a dial indicator, changed wheel bearings (set them up properly) and I still have the problem. I can pump the pedal to get it hard on the garage floor and then grab hold of one of front wheels and then rock the wheel / the whole car and when I then push the brake pedal it's noticeable softer. Could the deflection resulting in pad knock back be caused by the smaller drum brake spindle diameter and the smaller bearings that the drum brake spindle got? What also got my attention is my 8" wide rims with little backspace putting more leverage and force to the spindle and bearings. Could new spindles with lagers dim bearings offer less defection and keeping the brake pads closer to the rotor even on rougher road surface and tight cornering where I normally feel problem being largest?

here is a picture of my set up.

Any pictures of the bleeder location?
I see the brake line entering the caliper 'fairly' high.

Here is a picture of the caliper that I'm using, bleed screw is located higher and on the other side of the caliper that where the hose is attached.