Brake Problems

[Skip Carden]

I have a 73 Duster 340 with a big cam which generates very low vacuum I recently changed it from power brakes to manual. My problem is that you have to pump the pedal to get a full pedal but it will not lock up the wheels. I have changed the master cylinder 3 times and the same results. I have new front calipers and new lines anyone had this problem??

 

[Inertia]

Usually having to pump brake, is the first pump is moving something, the second pump applies pressure.
It's usually the rear brake adjustment, the first pump moves the wheel cyls/pushrods/shoes out to the drum, second pump applies pressure.
The front pads, if installed correctly, won't have movement as such cuz they just relax, no spring return.
We're I you, I'd adjust the rear brakes first, just checking all wheel cyl pushrods are in proper position on the shoes, that should get rid of the need to pump.
Master/cyl bore size, and proper bleeding can also affect pedal "feel".

 

[Oldmanmopar]

Make sure the pedal is releasing completely and the brake light switch is not stopping it from coming all the way out. If it doesn't return all the way it won't let it grab more fluid. Also the master cylinder should be a 1" or 15/16" piston. A 1 3/16" is for power brakes

 

[67Dart273]

To add to above, in no particular order
1....Master piston may be too small relative to calipers/ rear cylinders
2....Rear shoes may be out of adjustment
3....Air in system
4....On a side note, be sure the differential switch piston is not locked/ stuck/ shifted because not enough pressure (such as when bleeding) in half of system.

 

[Ricks70Duster340]

I recently changed to a larger piston MC and found my pushrod was too short to fully bleed the MC. The original was of questionable heritage so I got a new pushrod (previous owner said the front disc brakes were off of a non-Mopar). The new pushrod allowed full stroke on the MC to bleed all the air out of it. Now my pedal feels right and the brakes work better than ever.

 

[Oldmanmopar]

The smaller the piston in the Master cylinder the more pressure it will apply with less effort from the pedal. That is why never use a large piston Master for manual brakes. Read this in full and check Master cylinder applications for disc brakes.

I used a aluminum Master from a Dodge Ram with a 15/16 piston. pictured below My manual brakes worked like power. I could stop my car with ease at any speed.

Read this
V
http://www.moparts.org/Tech/MoparAction/Master_Cylinder.pdf



I typed this into google;

(dodge ram manual brake master cylinder piston size)

This is what came up


People also ask


What size master cylinder bore for manual disc brakes?


Manual Brakes. If the brake system is manual (no power booster), the master cylinder bore diameter should be less than 1 in., otherwise a very hard pedal will result. A power brake master cylinder will typically feature a larger bore diameter than a manual master cylinder.

 

[Sdriche]

make sure the pushrod length is adjusted correctly if it's adjustable. There's a tool to ensure you have the right length. Otherwise you may not be getting full travel.

 

[Skip Carden]

Thanks to everyone for the suggestions! I will try the rear brake adjustment first then check the push rod I have suspicions about it I think that the brake light switch is holding the master cylinder is not fully retracting it is a 1"one. Again many thanks for the help!! Skip

 

[harrisonm]

Let me add one more thing. you may not be bleeding your brakes properly. I have a friend who insists that the best method is for one person to pump, pump, pump on the brake pedal until the pedal feels firm, and then the guy opens the bleeder valve. That method has NEVER worked for me. It does not even make sense to me. In fact, we have bled brake systems together several times. The first time was his 67 Mustang. After trying his method for about an hour, I INSISTED that we use mine. 20 minutes later we had a firm pedal.
I believe in this method:
1. Start with the wheel furthest from the master cyl.
2. Make sure the MC reservoir is full. And check it frequently during the bleeding process. If the reservoir gets low, and you start sucking air, you start over.
3. Open the bleeder valve and say, "Down". Your assistant will press the brake pedal to the floor and leave it there.
4. Close the bleeder valve and say, "Up". Your assistant will release the brake pedal, let it rise to the top and leave it there.
5. Repeat steps 4 and 5 (KEEPING A CLOSE EYE ON BRAKE FLUID LEVEL) until the fluid is clear with no bubbles. Then do the other 3 wheels.
I believe this is the best method. It pushes clear fluid from the MC all the way to the bleeder valve.