From hard to soft brake pedal after short drive?
#2496788
05/16/18 10:55 AM
05/16/18 10:55 AM
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Joined: Jul 2013
Posts: 84 Norway
cudaerik
OP
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OP
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Norway
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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?
1970 Plymouth Cuda, 512cid on 230 400 block, 5 speed manual.(TKO 600) 1970 Dodge Charger 500, 446cid, 5speed manual.(TKO 600) 1969 Plymouth Roadrunner, 383cid, 5speed manual (Tremec TKX)
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Re: From hard to soft brake pedal after short drive?
[Re: cudaerik]
#2496940
05/16/18 08:06 PM
05/16/18 08:06 PM
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Joined: Dec 2014
Posts: 5,763 ohio
ruderunner
master
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master
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Posts: 5,763
ohio
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Rotor run-out can also knock the pads back
Angry white pureblood male
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Re: From hard to soft brake pedal after short drive?
[Re: ruderunner]
#2496982
05/16/18 10:36 PM
05/16/18 10:36 PM
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Joined: Jan 2003
Posts: 7,538 Albany, NY
67SATisfaction
The member whose name is actually Art
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The member whose name is actually Art
Joined: Jan 2003
Posts: 7,538
Albany, NY
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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
65 Satellite hardtop 361/4bbl console 727 2.76 67 Satellite convert 383/2bbl column 727 3.23 67 Lancia Fulvia Sport 1.3 Zagato. Alloy body, 1.3L V4 DOHC 4-spd 67 Lancia Fulvia Rallye 1.3. Alloy panel, 1.3L V4 DOHC 4-spd 71 Alfa Romeo GT Junior 1300 Zagato, 1.3L 4cyl DOHC hemi 5-spd 82 Alfa Romeo GTV6 2.5L SOHC hemi V6 5-spd transaxle 75 Maserati Bora US spec 4.9L DOHC hemi V8 5-spd ZF 77 Maserati Khamsin Euro spec 4.9L DOHC hemi V8 5-spd ZF 07 Aston Martin DB9 6.0L DOHC V12 6spd transaxle
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Re: From hard to soft brake pedal after short drive?
[Re: 67SATisfaction]
#2497361
05/17/18 09:21 PM
05/17/18 09:21 PM
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Joined: Nov 2004
Posts: 25,050 Texas
GoodysGotaCuda
5.7L Hemi, 6spd
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5.7L Hemi, 6spd
Joined: Nov 2004
Posts: 25,050
Texas
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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.
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Re: From hard to soft brake pedal after short drive?
[Re: cudaerik]
#2497380
05/17/18 11:00 PM
05/17/18 11:00 PM
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Joined: Jul 2005
Posts: 757 Toronto, Ont, Canada
boydsdodge
super stock
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super stock
Joined: Jul 2005
Posts: 757
Toronto, Ont, Canada
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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.
Last edited by boydsdodge; 05/17/18 11:02 PM.
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Re: From hard to soft brake pedal after short drive?
[Re: cudaerik]
#2498035
05/19/18 10:45 PM
05/19/18 10:45 PM
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Joined: Dec 2014
Posts: 5,763 ohio
ruderunner
master
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master
Joined: Dec 2014
Posts: 5,763
ohio
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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.
Angry white pureblood male
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Re: From hard to soft brake pedal after short drive?
[Re: astjp2]
#2500570
05/26/18 07:53 AM
05/26/18 07:53 AM
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Joined: Jan 2003
Posts: 1,376 NORTHERN CA
HUSTLESTUFF
pro stock
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pro stock
Joined: Jan 2003
Posts: 1,376
NORTHERN CA
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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
"Were in it to win it. Anything less will end up being..... A whole lot of fun doing!!" UNLAWFL RIP UN
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Re: From hard to soft brake pedal after short drive?
[Re: ThermoQuad]
#2502337
05/30/18 05:35 PM
05/30/18 05:35 PM
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Joined: Sep 2007
Posts: 14,889 up yours
Supercuda
About to go away
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About to go away
Joined: Sep 2007
Posts: 14,889
up yours
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Using a dial indicator check the end play on the rear wheel bearings. Set it to 0.0050 inches That's a bit tighter than stock, stock spec is .008*.018" 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 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.
They say there are no such thing as a stupid question. They say there is always the exception that proves the rule. Don't be the exception.
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Re: From hard to soft brake pedal after short drive?
[Re: Supercuda]
#2502340
05/30/18 05:44 PM
05/30/18 05:44 PM
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Joined: Aug 2003
Posts: 2,466 Answering the call of the wild
ThermoQuad
top fuel
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top fuel
Joined: Aug 2003
Posts: 2,466
Answering the call of the wild
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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
Last edited by Dilbert; 05/30/18 05:47 PM.
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Re: From hard to soft brake pedal after short drive?
[Re: ThermoQuad]
#2502346
05/30/18 05:54 PM
05/30/18 05:54 PM
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Joined: Sep 2007
Posts: 14,889 up yours
Supercuda
About to go away
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About to go away
Joined: Sep 2007
Posts: 14,889
up yours
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Take the fsm and ignore it as 22+ years of laps says 125 INCH pounds
You leave that at 125 inch pounds and you'll be putting in bearings sooner rather than later
They say there are no such thing as a stupid question. They say there is always the exception that proves the rule. Don't be the exception.
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Re: From hard to soft brake pedal after short drive?
[Re: Supercuda]
#2502560
05/31/18 07:04 AM
05/31/18 07:04 AM
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Joined: Jul 2013
Posts: 84 Norway
cudaerik
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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?
1970 Plymouth Cuda, 512cid on 230 400 block, 5 speed manual.(TKO 600) 1970 Dodge Charger 500, 446cid, 5speed manual.(TKO 600) 1969 Plymouth Roadrunner, 383cid, 5speed manual (Tremec TKX)
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Re: From hard to soft brake pedal after short drive?
[Re: cudaerik]
#2502561
05/31/18 07:16 AM
05/31/18 07:16 AM
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Joined: Jul 2013
Posts: 84 Norway
cudaerik
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here is a picture of my set up.
1970 Plymouth Cuda, 512cid on 230 400 block, 5 speed manual.(TKO 600) 1970 Dodge Charger 500, 446cid, 5speed manual.(TKO 600) 1969 Plymouth Roadrunner, 383cid, 5speed manual (Tremec TKX)
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Re: From hard to soft brake pedal after short drive?
[Re: BigBlockMopar]
#2505083
06/06/18 06:05 AM
06/06/18 06:05 AM
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Joined: Jul 2013
Posts: 84 Norway
cudaerik
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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.
Last edited by cudaerik; 06/06/18 11:48 AM.
1970 Plymouth Cuda, 512cid on 230 400 block, 5 speed manual.(TKO 600) 1970 Dodge Charger 500, 446cid, 5speed manual.(TKO 600) 1969 Plymouth Roadrunner, 383cid, 5speed manual (Tremec TKX)
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