soft brakes and brake light comes on-help[

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john27pa

john27pa

74 360/904 Duster
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Ok- 74 duster with disc up front and 10 inch drums in rear. I am having issues with my brakes being soft. I put on a new-rebuilt- master cyl about last summer?? Have good brake pads and brake shoes. No visible leaks anywhere and fluid doesnt seem to be going down at all. Bled the brakes-again- this past weekend. Helped very slightly-may just be the feeling of wishful thinmking also. They are power brakes. Im guessing either a bad power brake booster or somehow still have air in the master cyl.
Any ideas? Would a bad power brake booster give you the feeling of soft brakes? Making the brake light come on? I dont seem to get any air out of the lines when bleeding them.
 
Bad booster will not bring the light on, "soft" is a matter of perception with a power booster. They don't give you the "rock hard" feel of a manual pedal.

Bad booster also will not prevent fluid from reaching various points.

1--Did you bleed the master. Very first step

2--When you bled them, you should have started with the right rear (furthest away). Did you get fluid out of there?

3--You can "crack" fittings along the way ---I'd start with the master cyl. side of the prop/ warning switch and see if you have fluid to that point.
 
Are the bleed screws located at the top of each caliper? If not, then they are trapping air in them. Maybe previous owner bled them while removed from the mounts so he could move the bleeder screws to the top and then he reinstalled them on the mounts?

If this is the case, then the calipers were probably installed on the wrong side ( i.e. left on right side and right on left side).

Did this situation just come on suddenly? Did you just change something on your brake system?
 
Brakes were bled properly and in right order. Calipers and wheel cylinders are all in right direction. I get fluid coming out of all the right places lol.

If not the power booster then either mc needs bleed again or??

Just didnt know if proportioning valve or brake booster could be a culprit. Maybe just mc.
 
There's a quick, easy way to check for air.. - after you're sure the rear brakes are adjusted properly,

with m/c cap on,, have friend pump the brake pedal 2 or 3 times and hold down,,, remove the cap,, have the pedal released as you watch the fluid levels,... any "geyser" ( fluid spouting well above the surface) is compressed air, "springing" back,, front or rear,, and that system needs more bleeding..

don't mistake a small "spit back" as opposed to a longer "geyser"..

perhaps do the test to a vehicle you know doesn't have air,, then you'll see the difference..

my mentor took me to 3 vehicles on the lot without air,, then took me back to the vehicle we were working on.. geyser..

please note - if the rear brakes are outta adjustment,, the return springs can cause a fluid return from the rears.. setting the emergency can help if you can't adjust the rears..

hope it helps... saved me a lotta tail-chasing... grant
 
I've never liked or had much luck with rebuilt MC's. You can try clamping the brake hoses off and see how the brake pedal feels. If it's still soft/spongy, I'd get another MC
 
When you are bleeding, is the master cylinder at an angle? It should not be. It needs to be completely horizontal when bleeding or you'll never get all the air out. Most of them mount so the nose is up in the air. You need to jack the car up in the rear until the master cylinder is level and then bleed it.
 
Have you tried adjusting back brakes? If everything is tight and bled good as you say, it sure sounds like excessive shoe travel. Too much fluid flow will cause the switch to close giving you the light, just like a wheel cylinder letting go. My .02 cents.
 
I would suspect the remanufactured master cylinder is leaking fluid past one of the cups not allowing front or rear circuit to full pressure. I have had terrible luck with remanufactured hydraulic brake parts over the years including the ones I did my self.
Where these cars have readily available new manufacture master cylinders, and wheel cylinders for a few bucks more than rebuilt why would anyone not get the new stuff?
 
I've seen more than one reman'd master cylinder fail either during or shortly after bench bleeding. Not sure why.
 
Did you bench bleed the master before installing? Kind of an important step which many people forgo for various reasons. When you don't do it, you almost always have to re bleed it. You can get a cheapo bench bleeding kit at the parts store and do it while its still on the car. Just need a helper to pump the brakes while you watch the fluid. Can be messy so get some rags and cover up any paint that can be affected by splashing brake fluid.

The brake light comes on because there's a difference in pressure on one side of the system. There is a little valve or check ball in the brake switch. A pressure differential makes the valve move to one side or the other where it makes electrical contact and turns on the light. Almost always air or a leak.

Usually, a bad booster will give you a rock hard pedal with barely any travel. Lack of power assist is just that - no power assist.

You could take the line off the booster and check it (booster) with a vacuum gauge to see if it holds. If the booster is leaking vacuum, you can sometimes hear it under the dash, kinda like a squeak.

You could possibly have a leaking diaphragm inside the booster. If so, you may not see any external leaks but fluid can sometimes get into the chamber behind the master. You can try to disconnect the master from the booster (with the lines still connected to it) and see if there's fluid around the back seal or leaking down the inside of the firewall.

I would try gravity bleeding. Take the cover off the master and crack all the bleeders open. Make sure they start dripping freely. Go have a sandwich and and re check afterwards. While the bleeders are open just make sure to keep the master full. Brake fluid gets slippery, put something under the bleeders to catch the fluid so you're not tracking it all over.
 
Alot of good ideas and I will check those once the rain stops...someday. lol. Now I have a tapping noise that is very noticeable when I give the engine gas. I hope its not about to throw a rod. Doesnt maker any sound when I let off the throttle.
Im starting to really think about buying a 73 charger that a guy has locally for 3-4000. I hope this one lasts till then.
 
I'm with ramchrgr. I have read that if you don't "bench-bleed" the MC first, you will never get all the air out of it. You don't have to remove it, indeed I always do it once installed. You just need to disconnect the 2 tubes and install special recirc tubes back to the reservoirs. This is so important that most rebuilt MC's come with plastic fittings and tubes in the box. It may take 50 pedal strokes to get all the bubbles out (see youtube). Afterwards, you might just connect the tubes and have good brakes, but if bubbles made their way into the tubes, you will have to bleed each wheel. When swapping MC's, you can usually get by just bleeding the MC itself, though if using anything buy DOT 5, you should renew the fluid every few years anyway, to stop corrision.
 
I thought that red "BRAKE SYSTEMS" light meant my brakes were working under hard braking. Little did I know, it meant I had no brakes till I hit a tree going backwards at 5 mph. Later I found out the reservoir for my front discs was empty. I filled it up and you could see the MC start to leak weeks later. It was almost like the MC was sweating through its pores.
 
The brake warning lamp comes on when the pressure imbalance switch in the combination block senses a difference between the front & rear circuits. This exists in cars ~68+. I think the same lamp also lights when the parking brake is on.
 
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