Hello

Well I did the disk conversion on my truck. Everything on the truck is new in the front except the prop. valve and master cylinder... more on that later. Now rotors, calipers, pads and bearings are brand new. Along with brake lines and charger front brake hoses. The in the rear new pads and drums and components but not the wheel cylinders yet.... I converted to manual disk because I prefer the feel but this feel isn't right. I put in all these new parts and checked for leaks nothing. Bled everything really good. Still not right so I changed the prop. valve still bad. Changed the master cylinder and it didn't get better and its getting very frusterating. Now I will try my best to describe the pedal feel. If I push the pedal to the floor it will only slowly bring the truck to a stop. If I pump it once the just lightly use the brakes, I will get AWSOME stopping power like it should have. So why do I have to pump the brakes. The rear wheels are properly adjusted and bled well. Also the rear wheels will usually just lock if I dont pump the brakes with little action on the front end. It seems as if the prop. valve isn't working and there isn't enough pressure created from the master cylinder. Everything seems bled well. I've had the same problem for a while now. Any ?'s just ask. Thank you greatly.

 

I'm sure someone else will ask this, but did you switch to a dual chamber MC?

 

Is it a dual master for disk brakes? They are different between disc/drum and drum/drum systems.

If a proper dual master is being used, are the lines connected to the proper port on the master? The large reservoir should be plumbed to the front brakes, it is usually the REAR port on the master.

 

Was the new M/C properly bench bled?

 

Yep, classic master cylinder problems.

 

You used the same size metal tubing for both master cylinder lines, right? If you used a larger diameter tube for the 'large' fitting, well, there's your problem.
How do I know this? Uhhhh....

 

Yup I agree with ddavidv.

You see Balance is the prime requisite when dealing with hydraulic systems. If there is imbalance there must be a control for the imbalance. Ergo proportioning valves AND correct Residual Valves are used to "blend" front disc & rear drum systems together.

A residual valve holds a tad of pressure against rear, drum type, wheel cyls so they do not completely relax thus requiring being refilled, to be extended to shoe X drum contact (more than a partial pedal stroke to inflate/ fill & perssurize them). Once they are filled there's resistance in system & PSI can build causing brakes to apply hopefully equally to all wheels.

Double stroking or "pumping" on the brake pedal eventually makes up for in equalities, but stopping power may not be even/ equal when in a dynamic or motion situation in spite of how the pedal feels in a static situation. . . .

If everything is correct, except for some varied hard line IDs, the system still will not equalize and the vehicle will not stop correctly either, because pressures and volumes are not equalized and there is no control for that inequality.

A disc brake Master Cylinder is not the same as drum brake M-Cyl. And dual chamber is the only way to go if you want the best performance overall from the system.

But get it all dialed in and set up correctly & WOW! What A difference!

HAPPY Stopping Haaser!
FBp

 

Hello

Despite the popular belief that the brake lines are diffrent they are all 3/16 and the big chamber of the m/c to the front, the small to the back. Also the first master cylinder was from a 76 along with the prop. vavle.The second set of master cylinder and prop. valve was from a 78. I changed to a disk/drum dual master cylinder stock off the 73-79 fords. Now I haven't changed the wheel cylinders in the rear yet. Is there any possiblility that these old things are causing the problem. They are leaking a little bit but that shouldn't affect the braking performance should it? Also is there any special way to bleed the brakes? And finally would the wheel cylinder in a 2" braking system be smaller that 2.5" braking system which might cause rear wheel lockup? Any info need I can give you, I'm stuck. This isnt happening to other people i'm assuming? Thanks P.S It was properly bench bled too.

 

Sounds to me like there's still air in the system.

As far as the line size, a given volume of fluid in one end, the EXACT same volume out the other end, regardless of diameter.

Leaking cylinders will draw air into the lines, so change them out, and re-bleed the system.

 

I do believe that the master cylinders are different for manual versus power disk brakes. I used a master from a 69 f100 with manual disk brakes. Whether this is your problem is unclear.

It seems to me that some remaining air in the lines could still be an issue.

You ask about a special method to bleed brakes -- none that I know of. But the classic approach is to have a buddy, some hose, a couple clean containers and plenty of brake fluid. You attach the hose to a given bleeder, with the end of the hose in a partially filled contained of fluid. You open the bleeder and have buddy pump in and hold it, then you tighten bleeder and he lets out on pedal. Repeat until no air comes out, but don't forget to add back fluid in the master -- forget that and all of a sudden you are back to square one.

The other approach is less perfect, but easier. Just leave the bleeder open and pump the brakes. Make SURE the hose is always in the fluid so that if it draws back fluid, it will not draw air. This will get air out of the cylinders in the brakes, but may not get out little air pockets here and there in the system. I think it is traditional to start with the less perfect approach and then finish with a couple pumps using method #1.

If anyone else has better bleeding advice, speak up. I am no pro here, but he asked. We have assumed that he was bleeding them correctly.

 

If there is brake fluid getting on the rear shoes from the leaking wheel cylinders that can cause poor performance and lockup. I also believe the wheel cylinders are different size for the newer style rear shoes, that might cause some unequal braking(front to rear).

 

Those Mighty-Vac bleeder systems are pretty fun and easy to use

My manual method is very similar--have a friend pump up the system, hold down the pedal, and open the bleeder. Conventional wisdom is to start with the wheel farthest from the MC (right rear) and go to the next closest, and so forth. This might be an old wives' tale, but I'm not sure.

 

If they are leaking they need to be replaced. Pointless to go further until this is fixed.
I used a manual brake, disc brake master cylinder on my conversion from a 1970s pickup with the rear brakes new but 'original' with no problems. Bleeding was a snap.

 

What did you use for a proportioning valve? I was told that is you use the Ford disc brake one, you have to hold the little valve open on the prop. valve while you bleed brakes. This valve holds pressure on the shoes instead of releasing it immediately.

Not tried and true by me, just something I heard.

DDav is right, tho, you should change out those rear leaking cyls before you go further. If they are leaking fluid out, they could be leaking air in to the lines.

RF

 

Bench bleed the MC. Replace the rear cylinders. Install the special tool on the prop valve that bypasses the pressure loss valve and bleed the brakes. Gravity works fine for the initial bleed. Often that is all that is required. The special tool is a a spring clip that holds the little pin on the prop valve out. You can get by without this tool by releasing pressure in the front at the same time as the rear.