Hey guys, just wondering if someone can give some insight with my brakes.

Info:
1971 1200 Ute
Dual master cylinder(originally single master)
120y discs up front
standard diff
new pads,and shoes

Problem:Occasionally when I was driving(on single master) I would engage in brakes and the pedal would hit the floor without any stopping power. After a few pumps the pedal comes back to life and I have brakes again...VWHA-LA!...Anyway, what I got told by a local mechanic that disc brakes use more fluid than drums,so I needed to upgrade my master...I upgrade the master to dual system allowing the front and back to have their own resiviors, I salvaged this master off a ute I previously owned(was working fine in that ute). Same problem still occurs...After close inspection I see my left wheel cylinder has a leak. I then replaced both left and right cylinders...I bled the brakes then take it for a spin and the problem still occurs...They hit the floor with no power and then after a few pumps the come back to life. I have no idea why this random loss of brakes keeps happening. Any advice would be highly appreciated, cheers! **Forgot to add, a mechanic said that my drum assembly on the rear has the trailing/lead shoes the wrong way around...

Master Cylinder seals are toast one from wear and tear the second one likely corrosion from sitting around unused for a while are the only causes of such behaviour.

New Master cylinder or a very close inspection for corrosion and a new seal kit, I would do all the seals at the same time rear brakes as well and make sure the caliper pistons move freely and if not strip and clean and or replace front barke seals as well .

Generally though front calipers do not leak just seize up which means firm pedal and no brakes. so loss of pedal is related to both Rear Brake cylinders and or master cylinder.

So it was disc brake front all along with both masters?
My guess would be the front discs have too much runout causing them to push the pistons back into the callipers. When you apply the brakes the first time all it does is push the pistons back but there is not enough fluid to push them all the way to the disc, which means another application is necessary.
Do you feel a pulsation through either the car or pedal when braking?
Solution is to machine your rotors.

Both sound possible

* If the pedal pulsates, fix that problem (reface the rotors and clean the mounting surfaces)

or

* If the pedal doesn't pulsate, replace the master cylinder. So many new MCs these days are defective right out of the box.

I had a similar problem and it turned out to be the rear drum brake adjusters were buggered/stuck.
It took 2 -3 presses on the brake pedal to get the pads to meet the drums then next press gave me stopping power.
Drove me nuts trying to find the problem, bled the system about 5 times all seemed fine until after driving it for awhile.

I've had this happen on a friends 1200 coupe & it was worn seals in the master cylinder itself, not the front brakes. Replace the master cylinder with a new one, 11/16ths or 13/16ths, depending on the size of the rear wheel cylinders. Match the size of the M/C to the size of the rear wheel cylinders. Make sure the master cylinder is to suit front discs, a 1200 ute usually has front drums & the front circuit has a residual pressure valve that'll cause the front brakes to drag & sometimes even lock up. So, for the 11/16ths, ask for a Datsun 1200 Coupe Master cylinder. For the 13/16th's try for an early 120Y Master Cylinder. You could also ask for a Sunny M/C, but you'd have to install the rod that goes to the pedal. Also, when bleeding the brakes, don't forget to bleed the master cylinder itself. just run a clear line from the bleed nipple back into the reservoir, then pump the pedal a few times til the air bubbles go away. Brakes are one of the things I don't skimp on, because not being able to stop the car is a nasty feeling, & you sometimes don't get a chance for that second pump on the pedal.

New situation...New problem

Front brakes are staying locked after 2 minutes of driving
11/16th Master Cylinder Brand new (JB1196)
120y Discs
No air in the system,bled it 4 times. BOTH wheels get locked. After I crack the bleeder valve on ONE caliper, both wheels are unlocked. I understand it's probably a master cylinder problem. But I'm stuck on how to fix this, it wasn't happening with my old master.

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Had this happen to me as well. The rod between the master cylinder & the pedal is too long, so the brakes are dragging & overheating. Try to shorten this, some Master Cylinders have a thread & a lock nut. Or you could try swapping the rod from the old master cylinder. Or adjust the pedal stop so the pedal comes out a little. You could also slip some of those 'half moon' spacers between the master cylinder & the firewall. They are usually hard to find after putting them down, like pens or markers. If none of this is the case, it could be they have given you a brake drum master cylinder, which has a residual pressure valve, which will keep pressure in the front circuit.

Yes, that's the most likely causes.

Adjusting the pedal height/rod length is critical.
* For a custom master cylinder setup, it is trial-and-error.
* For the stock setup, back off the pedal stopper, then add or remove Master Cylinder shims until the pedal height is 143.5 mm. This assures the Master Cylinder won't stick. Then adjust stopper for pedal height to 141.5 mm.



Residual Valve