soft brakes and headaches

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Jp5

Jp5

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gents,

got a soft brake situation i could use some feedback on. i recently did a disc conversion up front on my 65 valiant and i can't get that good bite. whatcha think?

installed:
Scarebird conversion kit (front only)
single piston calipers
new master cylinder and 7" power booster with vacuum from intake manifold
new 3/16 tubing from disc/drum prop valve directly to front flex hoses
10lb residual valve to rears

I bench bled the master cylinder and have now bled the system 4 times. No leaks, no bubbles.

If i give it a lot of leg i can get the rears to lock up but that's full on pressing as hard as i can.

under full brake effort the thing stays perfectly straight too. no pulling either way
but i don't feel any nose dive at all.

wrong prop valve? should i get rid of the prop valve?

thanks in advance for your thoughts.

jp
 
you can see what i've added here
 

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Take a picture of the installation of your calipers.
Are the bleeder screws on the calipers at the top of the caliper, at the highest point?
 
what is the size of the master cylinder and caliper pistons?
.. larger than 1" will require a lot of leg
 
It looks like you have the multiplier linkage under the hood. Do you have the matching ratio pedal inside the car? Are all 4 tires the same size or? Is the residual valve correctly installed?
- Some A-body Disc/Drum cars had a metering block installed in the disc side to slow the fronts to allow the rear shoes to go out to the drums first.
-did you properly adjust the m/c pushrod?
- and adjusted the rear brakeshoes
-Could it be that the rear cylinders are too large and all the fluid is used just to get the shoes out?
 
I have found the following street set-up very satisfying;
-the oem KH discs with braided lines and 10 x 2 drums
-the multiplier booster with a 15/16 m/c
-7/8 wheel cylinders
-no metering block and a gutted p-valve
-tires are 235/70-14 and 295/50-15 rears
- Car is at 3450#
-I didnt gut the p-valve until I put the 295s on. I found that tire size to be a very effective stopper back there.
-I found the smaller m/c to be a little more modulate-able
 
If you have brake lines that go up and down I bet you have air trapped in that you can't get out unless you power bleed. My new favorite power bleeder is a 1 gal lawn sprayer. Empty brake res, put brake fluid in sprayer and adapt to bleeder screw or port and pressure bleed backwards. Air goes up way easier than down and works like a charm.
 
Say, bOb; You are of course right about the residual valves, but they can help prevent the wheel cylinder cups from relaxing and leaking.And if correctly installed, they shouldnt hamper the rears from working. And since its already in there...... Am I wrong? I have ONE installed in the rear line only.Its never been a problem for me.
And to Jo; If you look carefully you will see that the only line with a possible problem is the one with the elevated loop, and it goes to a front wheel, and hes not having problems with the front brakes. Furthermore he claims to not have a pull.And with the residual valve in the rear line acting as a check valve during bleeding its highly unlikely that he has a hydraulic problem. You are right about air liking to go up. Tickling the brake pedal will get that air out through the compensating port out too. Just takes a little more time.
-I believe the problem is mechanical.
 
newer wheel cylinders have springs to keep the cups loaded so a residual valve is no longer needed in the rear,since it is already there it will cause no harm so you can leave it
prop valve reduces rear initially so front operates first
 
Is the engine supplying enough vacuum to operate the booster ?
Power brakes without sufficient vacuum don't work well at all.
Easy test, disconnect and plug that vacuum line, drive it again.
 
hey thanks everyone. i appreciate your thinking on this.

ir333,
the caliper pistons are probably about 1" around. and the MC has 3/16 outlets for the fronts and 1/4 to the rear. the body of the MC is pretty big.


AJ, i'm using my original brake pedal as the booster/MC came with linkage and fit perfectly. i did not however do anything to the pushrod since my pedal is at just the right height. is that a critical miss? front tires are 225 and rear are 245's. residual valve is installed with the arrow pointing toward the brakes.

jo deisel,
i do plan on trying the vacuum bleed method. wouldn't that be great if it just sucked a big air pocket out of there and boom, brakes!

redfish,
i did wonder about the amount of vacuum the stock slant 225 provided. i'll give that test a try.

thanks a lot guys.
jp
 
-The slant has plenty of vacuum
-The pushrod has to be set just right; there is a procedure.If it is too long the compensating port may not be opening, in which case it would be impossible to bleed. If it is too short you may not be getting boost.If in between, but not right, then you could be getting differing results. So, yes, its important.
-Just because your stock pedal bolted up to the booster pushrod doesnt preclude that its right.
-You state that the caliper pistons are about 1inch; do you mean wheel cylinders at the rear? If yes than that could be the problem. The 1 inch bore requires a lot of fluid to move through to activate them. A lot of fluid means a lot of pedal travel. The front pistons are practically touching the discs and so require just a tiny bit of pedal travel to activate. So you begin to see the delemma.
-I think you should properly adjust the rears and try again. If still same, then remove just one drum and have a buddy apply the brakes a bit at a time while you are observing the wheelcylinders for action.If no action then problem may be air. If some action you will need to make a judgement call;ie is it enough to energize the shoes.A smaller wheelcylinder will provide more action for same amount of pedal travel.
-An easy way to tell if brake shoes are near proper adjustment is to apply the parking brake.If it requires a lot of lever travel then they need it.
-Also, while the rear wheels are in the air, you can do a sortof stall test. Apply the brakes (with drums and wheels installed). Then with engine running and trans in manual low,give it some throttle and see how much brake effort is required to prevent wheel rotation. If it really requires "full on",then something is definitely not right. Watch out for stones flying off the tires.
-Im still thinking the problem is mechanical in nature
 
thanks for the additional thoughts, aj. i'll make drum adjustments and see where it leaves me. in the meantime i'll look into that pushrod situation.
 
Heres a thought; isolate the fronts to see if you have a hard pedal.That is to say, disable the rears. There are several ways to do this. You can 1) pinch the rubber hose at the back( not recommended), or 2)plug the m/c output to the rear, or 3)remove the brake shoes and clamp the pistons into the wheelcylinders, or 4) disconnect the rear flexhose and cap the steel line. I have used all four methods. Method 1) is the easiest but it is possible to damage the line. Methods 2) and 4) will require re-bleeding. And method 3) will not prove accurate if there is air in the line somewhere.
- I would start with method 4) because; a)it does not req. re-bleeding and b) if the pedal gets hard you instantly know that the problem is in the rear mechanics.
-But if method 4) doesnt get you a hard pedal, then I would first observe the rear flexhose to see if it is expanding due to internal failure. Then if it is ok I would be tempted to use method 1)
-then if still no hard pedal, I would go to method 2)
-At this point you have totally eliminated the rear brakes and since you have already roadtested the car, the fronts are working, and the pedal should be hard.
-But if its not hard, the problem is in the front, or in the m/c itself.
-To prove the m/c just plug the front brake port as well. If still soft, I would bench bleed it one more time, with it being level to expel the air which can get trapped in the nose. This would be a good time to also prove the compensating port is working. So now if the pedal is hard you have a good m/c. But if still soft shes defective.
-Lets assume the m/c is good. Reconnect the fronts and bleed the line to the p-valve. Now the pedal should be hard. But if its not; theres either air in the front system or there is a mechanical problem. So again we observe the flexhoses.If theyre ok, the next most likely place is the interface between the calipers and the discs. So we have a helper pump the brakes moderately at regular intervals while you are observing the action. If the pads were correctly installed and the calipers are free to self-center, then there will be very little to see. You may be able to see the pistons moving back and forth a few thousands but thats all. If however there is more action, then thats the problem. To isolate the hydraulics I remove the calipers from their mounts and clamp the pistons into the bores with a big C-clamp. Now the pedal has got to be hard, unless there is air in the lines. If soft, re-bleed.If hard the problem is in either the pad to disc interface or the pad to piston interface. The face of the pad friction material has got to be parallel to the steel backing material. If the steel backer is bent it will act like a spring forcing the piston back into the caliper after every application giving a soft pedal. So will any silicon material pasted on there. If the pad is worn at an angle,or the pad is sitting on a worn rotor,the caliper will tend to rock. So fix the interface.Thats it we are done with the fronts, and the pedal is hard. So,reconnect the rears and continue trouble shooting; ie go back to the top of the post.
 
Heres another thought; if you tried method 3) above;clamping the pistons into the wheelcylinders, this is a great time to see how air in the lines can be proven.This is cool.
- Cover all underhood painted surfaces within about a 2 ft radius of the m/c and then uncap the m/c. Have a helper push the brake pedal with something like 20 or 30 pounds of force and hold the pedal down. Now you stand about 2 ft away from the m/c and watch it while the helper removes the force. If the system is free of air and the compensating port is working, and the pedal is hard,then you will see a little fountain of brake fluid returning to the reservoir,forced back by the contracting of the flexhoses.I cant say how high it will be; something like an inch. But if there is a gusher and a soft pedal then thats indicating the expansion of an air bubble somewhere, or really soft hoses, or springy pads. Now the more force on the pedal, the higher the fountain will be; so start gently if you value your paint.
- And if you do get a gusher,the hunt is on.
-And after you get a hard pedal you may still want to prove that the booster is working; but thats the last thing to do. HARD PEDAL is paramount.
-Ok its long past my bedtime. May the force be with you.
 
How far does the pedal go down? Are you aware that disc brakes do not have the stone pedal drums do? Also, changing from manual to power is another big difference. Power brakes always feel mushy in comparison. Couple that with discs and is it possible that's what you're feeling?
 
I did a disc brake conversion on my Barracuda a while ago. Couldn't get the fronts to lock up no matter how hard I pushed. It was because I didn't bed the brake pads into the discs. You need to do 10 or so hard stops with out locking anything up & without coming to a complete stop. Apparently you need to get some pad material onto the discs before they grab. Took me a while but now the brakes are great, I even removed the power booster.
 
AJ,
thanks again for all the pointers. isolating the fronts and the rears seems like a great idea. i'll add a few of these to my list for saturday morning brake trouble shooting.

rusty rat rod,
the pedal doesn't go super far. it certainly doesn't reach the floor. feels like a reasonable range of motion. it's not so much the pedal feel that i'm trying to fix rather than ultimate bite on the brakes. but i hear you on the differences.

OZ67,
i have bedded the pads yes. but i'll add some more hard stops just in case.
 
Hold on there BubbaLouie. I just checked out the Scarebird site. Theres not a lot of detail there. What are the rotor and caliper piston diameters? What knuckles are you using? That set-up doesnt look right to me.
-From the pics; the rotor and pistons look almost tiny, and the calipers are mounted very close to the spindle centerline.If I had to judge, by the pics,I would have judged them too small and stayed away from that set-up. And the lack of details also scares me.Ive seen parking brakes almost that size.
-Im inclined to think that the only thing wrong with your system is inadequate size for your application. I dunno.
- Anybody out there ever used these succesfully?
 
I've got brakes!
i readjusted the rears to remove as much extra play as possible and my pedal firmed right up. couldn't believe it. i guess all the fluid was going to that extra piston travel and not allowing a good clamp up front. man it felt good to lock em up on my test drive. thanks again guys for all the thoughs and pointers(aj, thanks for the extensive thoughts here. i'm very happy it was the easiest of them all!). i'll still fine tune that pushrod this weekend and hopefully can squeeze more power there. i'll let you know how that goes as well.

jp
 
aj,
as far as the scarebird sizing goes. the rotors just about fill the entire space behind my 14" steelies. not the biggest rotors i've ever seen but they don't strike me as too small for the car. i'll get you a pic of the set up with my wheel removed and you can have a look.
 
jp; if you can lock them up the scarebird size must be adequate, so pics not needed.
-also since the pedal got hard and you seem to have boost, the push rod must be very close. Just beware of a pedal that seems to get lower over time which might indicate a non-functioning compensating port, or rear brakes needing adjustment. Sounds like youre good to go, so enjoy.
 
AJ,
finally got to my pushrod adjustment and found more brake power still. so thanks again for the feedback.
 
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