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upgrading master cylinder to handle full Disc Brake Conversion (front&rear)


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Is there a lovely person out there with some sound advice on a master cylinder that would be compatible with a 77' 620 Datsun for front and rear disc brakes?


I have already stripped the drums off front and back. Replaced drums with mounted Calipers/pads/rotors. Have totally redone the brake lines (prop valve) from a new (rebuilt) master cylinder that I thought was compatible with a 620 off of a full disc 280zx. But my problem now is that the calipers/Pads in the rear aren't adjusting to the rotors once the I push the break pedal. So basically when I use the brake pedal the rear pads are having to clear to far to engage the rotor and it is taking all the pedal and they don't adjust. Also If I close off the line to the rear the front work just find and the pedal pressure is great!


So I am thinking I either need a longer Push Rod. Or different booster/master cylinder to compensate for the extra pressure and fluid needed for the rear, or possibly a proper Prop Valve (I just have a universal adjustable prop valve on there right now)


I realize how stupid I am now for attempting to put rear disc on this tiny thing and understand I might have to just reconvert to rear drums front disc but was hoping there is an answer out there.

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Guest Rick-rat

I have discs front and rear on my 73 620, used 280ZX 15/16 mastercylinder, has the small brake booster, no proportioning valve. Mine work great. Try bleeding your rear brakes again, pads should not need to go very far to engage. Are the lines to the brakes on the master cylinder in the right place? I.E. F and R markings on the master cylinder What brake calipers/rotors/ bracets do you have?

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You might try putting a 10lb residual valve for the rear brakes. It is a small valve that goes in the rear brake line and keeps a small amount of pressure in the line so that the pads dont retract so far. You can also put one in the front. look in jegs or summit.

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Master cylinder size depends on the calipers used. For example 280zx front calipers will need the 280zx 15/16" master.


Disc brake pads do not retract. They are always in contact with the rotor. The residual valve keeps 2-5 pounds of pressure on the pads to keep them clean. Ten pounds is too much and is better suited for drum brakes. If soft then air is trapped still. Did you bench bleed the rebuilt master before using? I just get an old brake line about a foot long and connect to the master and bend the end around and over into the master reservoir to recycle the fluid. Just keep pumping until no bubbles. Remove the line and connect up the truck brake line and proceed to bleed them..


The 620 doesn't have a proportioning valve but it does have a load sensing valve that allows more brake pressure to the back when loaded with heavy cargo. It has several bleeder screws that have to be bled in order.

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With drums, the springs collapse the wheel cylinder pushing the fluid back to the m/c and reservoir. Since there's no springs in a caliper, they don't push the fluid back to the m/c. The piston in the m/c will pull fluid a bit as the pedal is let off, but it doesn't typically create enough pull or fluid movement to pull the caliper back. I'd say set the e-brake, but I'm guessing you don't have cables hooked up....I've only seen one person that's bothered to hook them up with a rear disc conversion. You might try to find a way to activate the e-brake levers on the calipers, then re-bleed the rears. That way, there will be enough fluid in there and what ever is "pulling" the piston back won't be able to.


If you push rod was too short, you'd have a lot of pedal travel before it engaged the m/c piston. The spring on the pedal under the dash will hold the pedal up....you should only have about an 1/8" - 1/4" of pedal movement before the pushrod is fully seated in the m/c and starts pushing the piston. BUT!!!.... you must have at least a small amount of movement to make sure that the m/c piston is coming back all the way. If it does not come back all the way, it won't let the fluid back into the reservoir or let move fluid into the m/c bore.


I'm not familiar at all with the load sensing proportioning valve on the 620's.....since you have an adjustable in the rear circuit, I would eliminate the load valve. It may be the culprit.


Another possibility......the front/rear circuit positions changed around a bit from year to year, model to model on the m/c's. There should be an F and an R on the m/c....double check that your front brakes are connected to the F and the rears to the R.


I'm going to change up what Mike said on the bench bleeding. The point of bench bleeding the m/c is to get all the air out of the m/c.....all agree on that. But, the biggest reason they're bled on the "bench" is because they are not typically level when mounted in the car and the pedal assembly will not push the pistons completely in. I do the bench bleed on the car. I jack up the rear end just a bit to get the m/c level(on some cars, the m/c is at a severe angle, but not so on datsuns) and I take out the pushrod under the dash. It will take a second person to do it my way, which is another advantage to Mike's way. There's bleeders on the m/c, so you can have all your lines hooked up. Have a helper crack both bleeder valves open, then you can take a long screwdriver or dowel and push the m/c piston in until it completely bottoms out.....hold it there and have the helper close both valves. Let up on the piston. Repeat a couple of times, but there's no need to do it a dozen times. I go one step further and I also do it at the brake line connections under the m/c. Then I know I've got fluid coming into the lines and the air is removed. Just make sure the piston is not let up at all while the valves or lines are open. You can put the pushrod back in to bleed the rest of the system.


oh....and Mike's right about the residual valve, but they're really only ever used when the m/c is located below the calipers or wheel cylinders.....as in really old cars where the m/c is under the floor board, attached to the frame. They're used to keep the fluid from being drained back into the m/c. Using a 10lb on a caliper will cause it to drag quite a bit.

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