Help need to press brakes twice

[FatehShehryar]

I recently bought a datsun 510, the brakes seem to only work if you press em twice. It has had an engine swap (to a 4sfe) and a front drum to disk brake conversion. Can anyone point out as to what could be the reason that is; and yes the brakes work fine on the second press. Thanks

[datzenmike]

Are you using a single master cylinder for all the brakes?

Disc brake pistons are generally larger and require more fluid to move them then drum brakes. Usually a larger master cylinder is used to reduce pedal travel.

Air may be trapped in the front or rear hydraulic lines still and needs bleeding.

I don't know what disc brake calipers you have used but they are often left and right specific. Make sure the bleeder is at the highest point so the air bubbles can get out.

The rear drums are badly out of adjustment. This eats up pedal travel.

[DanielC]

Bleed the brakes.  In my experience, Datsun wheel cylinders only have one port in the wheel cylinder.  You cannot use a pressure bleeder, when bleeding the brakes, you have to let the brake shoes collapse the wheel cylinders to push the air in the wheel cylinder back out the bleeder fitting

[mklotz70]

This isn't to counter Dan or Mike, but here's another 2 cents.  If your brakes are spongy, you have air in the lines.  If they are nice and firm on the second press, then you simple have too much travel as Mike said. If you fronts are disc, then the pads should stay (pretty much) against the disc.....so very little travel.  Your rear drums are where the culprit will be.  You can pull the parking brake.  Pump them twice like you have been.  Now let them sit for a minute.  The next time you press them, they should not need a second pump.  Basically, having the parking brake engaged takes up the extra travel.  If they feel good doing this, then that shows that your rear brakes need adjustment or work.  If they still need to be pumped, either your parking brake isn't working, or you have an issue in the front.

 

As for Dan's info.....I'm not quite sure about it.  The pressure bleeder only works at about 3-7 psi...or at least they will.....which isn't anywhere near enough to move the springs and push the shoes out.  I agree they need to be compressed.....so don't try to do it with the parking brake engaged.  I've made my own pressure bleeders before and used them on the 510 wagon I had and the 521s.  Dan and Mike have some serious knowledge and they're way more active and current with this stuff these days.  Hopefully, between the 3 of us, you'll get if figured out. :)

[datzenmike]

That's good stuff Mike. I remember as a kid rather than adjusting the rear shoes I would pull the e brake a couple of clicks.

[FatehShehryar]

Thanks all I'll try what work and update you guys ?

[DanielC]

You need the movement of the pistons in the wheel cylinders to expel any air in the wheel cylinders.  Applying a constant pressure to the brake hydraulic system does not allow the wheel cylinders to move in and out to expel the air. 

If you use low pressure to pressure bleed, the return brake spring pressure is not overcome, and the wheel cylinder pistons do not move.  If you use high enough pressure to overcome the return springs, the springs only collapse the wheel cylinder the one time the pressure is removed.  This probably is not enough to expel all the air in the wheel cylinders.

 

 

[banzai510(hainz)]

Im to lazy to read everybodys responses but rear brakes need adjustment for sure. 510 in USA have a 7mm square peg that needs to be turned Inward and its a wedge shaped to spread the brake shoes apart to make up for wear in the shoe.You will feel like 1/4 clicks when turing the square peg

If your parking brake works and pulled, does the brakes work with 1 pump?If yes then you need to adjust the rear brakes. 510s in USA use a dual brake master cylinder. Others countrys might use only 1 cylinder or early models

 

Edited October 30, 2018 by banzai510(hainz)