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Unread 10-09-2013, 09:03 PM   #1
paulkeith
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Brake Bleeding - Pedal soft then hard

Another brake bleeding thread......

I replaced my proportioning valve because it was leaking at the small covered plug on the back end of the valve. Replaced with a standard PV-2 (universal disc/drum prop valve, talked to some vendors who confirmed they're all the same).

http://www.summitracing.com/parts/clp-pv-2

Pedal is soft for first ~1/3rd of travel, then rock solid. Every time. Pumping has no effect. Fluid squirts out of the front (small, rear brake) reservoir during the soft 1/3rd and stops when the pedal gets hard.

Open either a front or rear bleeder and the rock solid portion goes away and it just moves fluid out of the open bleeder.

Brakes will not lock at speed, but jeep will stop.

I have bled close to a gallon of fluid through the system. I can gravity bleed effectively at all 4 corners, nothing but clean fluid comes out gravity bleeding or 2-man bleeding. Bench bled the master twice.

Rear drums adjusted, just dragging on the drums.

Bad master cylinder? Still not bled? Prop valves aren't actually generic?

The vac booster was replaced by the PO not long before my purchase so perhaps pushrod length issue?

I am at my wit's end.

Any random thoughts, advice or suggestions much appreciated.

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Unread 10-09-2013, 09:19 PM   #2
LumpyGrits
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Well, IF you would fill out your profile.
Do you have 'power' brakes?
If so, you should check your booster for vacuum leaks.
Hope you used DOT 3 fluid, as that is what the system is built for and you don't want to mix fluid types.
When was the last time you adjusted your rear brakes.
LG
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Unread 10-09-2013, 09:23 PM   #3
paulkeith
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Fair enough -

'84 CJ-7, 4.2L, power brakes. new booster.

This behavior is with the engine off. Engine on, I get do get noticeable vacuum assist and the pedal just feels soft.

All DOT 3.

Rear brakes adjusted earlier today. Just dragging on the drums.

Thanks for the reply.
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Unread 10-09-2013, 09:27 PM   #4
LumpyGrits
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What you are see'n with engine off is normal.
You really should have said your findings, were with "engine off"
When you bleed the brakes. Don't let the pedal go to the floor. Make sure your 'pedal-pusher' keeps pressure on the pedal when he feels the pedal start to sink, when you open the bleeder.
You run'n big tires?
LG
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Unread 10-09-2013, 09:36 PM   #5
paulkeith
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LG - Thanks for the help....but I'm not positive I agree that some soft pedal engine off is normal....I've bled many other cars dozens of times and I get rock solid pedal at the top of pedal travel with engine off. I do not mean the small amount of slack before the MC pushrod engages. I'm getting fluid squirting out of the small front reservoir during that 1/3rd soft pedal portion which means pistons in the MC are moving...which means either the MC seals are bad or air is in the line somewhere, right?

Also as I mentioned it can't lock up the tires - I was able to lock the tires before the valve swap, albeit only the rears which I suspect was an issue in and of itself with the previous prop valve.

Thanks in advance again...
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Unread 10-09-2013, 10:40 PM   #6
Spieg8
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Most brake systems are designed so the rear locks easier than the front (makes it easier to steer straight in an emergency stop). Also the weight shifts to the front in a stop, so the rear loses a little traction compared to the front.

I believe it is normal for some fluid to fountain/squirt when the pedal is pressed and the MC cover is off.
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Unread 10-10-2013, 09:44 AM   #7
John Strenk
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Quote:
Originally Posted by Spieg8 View Post
Most brake systems are designed so the rear locks easier than the front (makes it easier to steer straight in an emergency stop). Also the weight shifts to the front in a stop, so the rear loses a little traction compared to the front.

I believe it is normal for some fluid to fountain/squirt when the pedal is pressed and the MC cover is off.
I dont agree with that. Most proportioning valves are designed to drop the pressure to the rear brakes and or delay the force so that the rears don't lock up.

I don't think it's easier to control a car when the rear end is swapping positions with the front.

As for the OP problem... Well i'm working on a brake problem myself.

Rock hard pedal engine off and sinks to the floor when the engine is started.

Pretty sure it's the vacuum booster.
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Unread 10-10-2013, 10:35 AM   #8
LumpyGrits
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Quote:
Originally Posted by Spieg8 View Post
Most brake systems are designed so the rear locks easier than the front (makes it easier to steer straight in an emergency stop). Also the weight shifts to the front in a stop, so the rear loses a little traction compared to the front.

I believe it is normal for some fluid to fountain/squirt when the pedal is pressed and the MC cover is off.
WRONG----The vast majority of auto brake systems are 60/40 split.
60% of braking to the front. 40% to rear.
Paul--replace the booster.
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Unread 10-10-2013, 04:54 PM   #9
86cj74.2L
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Try bleeding your mastercylinder then do the hole procedure over again.

I'm not a fan of pump the brakes then open the bleeder. All that does is take one air bubble and make 1 million little ones out of it.

Open bleeder and push pedal. Then close bleeder and let pedal return. Engine off first then engine on when no air bubbles with it off.

I bought the motion pro pressure bleeder. Will never go back to the two person bleeding again.
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Unread 10-10-2013, 05:26 PM   #10
paulkeith
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I have a motive power bleeder.....the vanco brakes guy told me via email that it is possible to damage proportioning valve seals and cups at anything over 8psi so I've tried to avoid using it this go-round.

Bought some plugs, will first confirm the master is good and then go from there.
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Unread 10-10-2013, 06:28 PM   #11
Spieg8
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Originally Posted by John Strenk View Post
I dont agree with that. Most proportioning valves are designed to drop the pressure to the rear brakes and or delay the force so that the rears don't lock up.

I don't think it's easier to control a car when the rear end is swapping positions with the front.
Exactly my point. Perhaps I worded it poorly. As the rear brakes lock a little earlier than the front, this assists the vehicle in tracking correctly. If the front brakes locked first, the rear end would tend to want to get ahead of the front (like putting the feathers near the tip of an arrow). The proportioning valve ratio is designed to balance out the braking force with the transfer of weight from center to front when stopping. The factory PV is set for normal driving conditions on pavement where vehicles travel at high rates of speed and controlled braking is critical. Even with the standard proportioning valve, the front will often lock before the rear in low traction environments (dirt, ice, etc...) due to the fact that the vehicle does not stop fast enough for the vehicle weight to transfer forward as much as it would on pavement.
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Unread 10-10-2013, 06:39 PM   #12
LumpyGrits
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Don't forget, that it takes higher brake line psi to operate disc brakes than it does shoe type.
That is also the work of the proportioning valve, on a disc & 'shoe' vehicle.
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Unread 10-10-2013, 07:48 PM   #13
paulkeith
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Spieg8 - Once an axle locks, directional control of that axle is lost. Locked tires have less traction than non-locked tires. If the rear locked first, you'd have no control over where it goes and therefore lose directional control of the vehicle, rear trying to pass the front. The proportioning valve is designed to reduce incremental pressure applied to the rear brakes in order to PREVENT the rears from locking up before the fronts. Ideally you lock all 4 at the same time, but if 2 have to go first, you want the fronts first, and then the rears act like the feathers you mentioned.

As for my original problem....I don't have a shining golden solution, but I do have improvement.

I replaced the master tonight, thorough bench bleed with the hose method and plug method both.

Just let it gravity bleed for a while and then took it for a spin.

Engine off the pedal exhibits the same sharp transition from soft to hard about 1/3 to 1/2 way down, but the soft portion is much (50%?) stiffer.

Engine on, the pedal feels nice and solid and the Jeep is very driveable. Took it to a parking lot and did some panic stops and a) I can get some tire squeal b) the rears are not locking prematurely.

Bottom line, the pedal feels better now than it has since I've owned it. Not perfect, but maybe I'm spoiled from other newer brake systems. So...i suspect either the old master cylinder may have been partially failed (or insufficiently bench bled...).

When I can score some time from the wifey I'll try to do a really good 2 man bleed and see where that gets me. Or maybe tonight with my trusty 2x4.
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Unread 10-10-2013, 07:57 PM   #14
LumpyGrits
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Quote:
Originally Posted by Spieg8 View Post
Exactly my point. Perhaps I worded it poorly. As the rear brakes lock a little earlier than the front, this assists the vehicle in tracking correctly. If the front brakes locked first, the rear end would tend to want to get ahead of the front (like putting the feathers near the tip of an arrow). The proportioning valve ratio is designed to balance out the braking force with the transfer of weight from center to front when stopping. The factory PV is set for normal driving conditions on pavement where vehicles travel at high rates of speed and controlled braking is critical. Even with the standard proportioning valve, the front will often lock before the rear in low traction environments (dirt, ice, etc...) due to the fact that the vehicle does not stop fast enough for the vehicle weight to transfer forward as much as it would on pavement.
WRONG again, you will loose all control.
Trust me here, I have driven semi-trucks for over 35+ yrs and you do NOT want your rear trailer brakes locking---EVER.
This was the big WHY that anti-lock brakes came out on passenger cars/trucks.
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Unread 10-10-2013, 09:43 PM   #15
Spieg8
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Sorry to belabor the point, but I hate to see misinformation.
I know this has strayed a bit from the original posting, so I'll offer this and then STFU.

Quote:
Originally Posted by paulkeith View Post
Once an axle locks, directional control of that axle is lost.
Exactly so... unless you have all-wheel steering, your rear wheels don't contribute nearly as much to directional control as the front. This is why early ABS systems had anti lock controls only on the front wheels.

Quote:
Originally Posted by paulkeith View Post
Locked tires have less traction than non-locked tires.
This depends on the kinetic friction value of the road surface. On clean dry pavement, the kinetic friction is usually close enough to the static friction so that locked brakes will stop faster than non-locked brakes. I remember watching old episodes of Motor Week when ABS systems first became a popular option, and vehicles with locked brakes always stopped in shorter distances than the non locked brake version of the identical car (exception being on wet or icy road surfaces where kinetic friction is lost due to hydroplaning). This is why race cars (NASCAR, Indy, Formula 1, etc...) do not use ABS... they need to be able to stop as fast as possible, and on pavement that means sometimes locking up the brakes.

Quote:
Originally Posted by paulkeith View Post
If the rear locked first, you'd have no control over where it goes and therefore lose directional control of the vehicle, rear trying to pass the front.
I'll repeat - your rear tires don't contribute to directional control as much as the front. On pavement, a locked rear tire gets better traction thus keeping the rear of the vehicle behind the front.

Quote:
Originally Posted by paulkeith View Post
The proportioning valve is designed to reduce incremental pressure applied to the rear brakes in order to PREVENT the rears from locking up before the fronts.
Not entirely true. While the PV does reduce fluid pressure to the rear brakes, it is only enough to prevent the rear brakes from locking significantly before the front. They will lock only SLIGHTLY before the front brakes. If the PV was not there at all, the rear brakes on most vehicles would lock much much too easily.

Quote:
Originally Posted by paulkeith View Post
Ideally you lock all 4 at the same time
If you really believe that non-locked brakes have better traction than locked, then why would it ever be ideal to lock all 4 wheels?

Quote:
Originally Posted by paulkeith View Post
but if 2 have to go first, you want the fronts first, and then the rears act like the feathers you mentioned.
Again, if this were true (and in most conditions, I believe it is not), I ask why would manufactures install ABS on the front wheels only (my 99 blazer for example only has ABS on the front wheels)? This goes back to the superior directional control of the front wheels (which would be lost if the front brakes are locked).
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