Brakes? What brakes?

[kidatari]

So, I was driving home during Lunch today, when I went to stop, pressed the brake pedal, and it went to the floor. I stopped on the side of the road, pumped the brakes, got pressure back and limped home. On the way home, the truck seemed to brake fine, I even tried panic stopping 3 times, worked without issue.

 

Now the truck has no brakes. Pedal goes to the floor, and returns by spring. No resistance.

- MC was full

- No moisture on any of the brake drums.

- No amount of pumping makes any difference.

 

I suspect that it's probably the MC, after not being able to find any split lines, cracked hardlines, leaking wheel cylinders, etc.

 

Anyone have any ideas? 1970 521, drums all around.

[datzenmike]

Loosen the E-brake adjustment then raise each wheel and adjust the shoes out till they just barely drag.

 

If that fails bleed the system to be sure that there is no air in it.

[kidatari]

Loosen the E-brake adjustment then raise each wheel and adjust the shoes out till they just barely drag.

 

If that fails bleed the system to be sure that there is no air in it.

 

Bleed is in the near future, but what is the thought behind the rear wheel ajustment? Seems more like a hydraulic issue to me, so I'm curious as to what you're thinkin'.

[datzenmike]

Brake shoes are pulled back away from the inner drum surface by springs when the pedal is released. As they wear down they have further and further to go before making contact. Multiply this by 4 X and the fact that the 521 only has ONE outlet from the master cylinder. If the shoes are not adjusted this will eat up pedal travel.

 

Disc brakes are much different. The disc pads don't retract and have almost no distance to travel, in fact they are gently rubbing the rotors.

 

 

 

Be a shame to replace the MC and then find you have to adjust them anyway. Just a thought.

[banzai510(hainz)]

have somebody hit the brake and you look at the brake resivoir. If bubbles come out the top its the Master cyl. There will usaully be NO leaks anywhere.I kept bleeding and bleeding to no avail. then put new MC bleed in like 3/4 strokes done.

I had this happen to my brakes after blleding them. I guess the rubber wore out after letting hit the floor a few times. The rubber hits a ridge inside bore and then tears and blows out.

 

Most 521s haver the single line

 

Rockauto has these but most are now Taiwan/italian units. Dont last more than 5 years seems like. If your going to keep the truck get 2 as I do.Believe me youll use it.

http://www.rockauto.com/catalog/x,carcode,1212310,parttype,1836

 

 

get a front and rear whelll cylinder also while your at it.

[kidatari]

have somebody hit the brake and you look at the brake resivoir. If bubbles come out the top its the Master cyl. There will usaully be NO leaks anywhere.I kept bleeding and bleeding to no avail. then put new MC bleed in like 3/4 strokes done.

I had this happen to my brakes after blleding them. I guess the rubber wore out after letting hit the floor a few times. The rubber hits a ridge inside bore and then tears and blows out.

 

Most 521s haver the single line

 

Rockauto has these but most are now Taiwan/italian units. Dont last more than 5 years seems like. If your going to keep the truck get 2 as I do.Believe me youll use it.

http://www.rockauto.com/catalog/x,carcode,1212310,parttype,1836

 

 

get a front and rear whelll cylinder also while your at it.

 

I was unable to try and bleed the brakes, as the wheel cylinder bleeder threads on the LT RR wheel were trashed. I'm ordering wheel cylinders and a Tilton 74-series motorsports master cylinder to get this sorted. Wish me luck :o

 

On MC Bore, if the stock bore is 3/4in., and I plan on going to discs shortly, should I upgrade to 13/16 bore? Thanks-

 

Josh

[Bleach]

Dont' forget to adjust your brakes still. The Datsun trucks did not have auto adjusting drum brakes. Even the later models with front discs still had to have the rear drums adjusted on occasion for best braking.

[MikeRL411]

Is your brake light activated by a micro switch on the pedal or by a pressure switch on the brake line distribution block? On occasion the pressure switch fails and has a small nearly invisible leak around the insulation around the electrical connector. It is, however, big enough to allow air in and fluid out and really screw up your brakes. The pedal will floor as you described. Keep your hand brake adjusted, it may be all you have when this switch fails.

[banzai510(hainz)]

just get a stock MC cheap 50$ or less.

as for the bleeder? just use a clear plastic hose and put over nipple lose the bleeded and pump. put tube at bottom a glass jar or bottole when you relase it will suck up the fluid. Nothing fancy but it works.

[yellowdatsun]

Changing the master cylinder bore size is usually not a good idea, unless you're changing the size of the wheel cylinders keep the stock bore size. This is one of those cases where bigger is not better.

[datzenmike]

I was unable to try and bleed the brakes, as the wheel cylinder bleeder threads on the LT RR wheel were trashed. I'm ordering wheel cylinders and a Tilton 74-series motorsports master cylinder to get this sorted. Wish me luck :o

 

On MC Bore, if the stock bore is 3/4in., and I plan on going to discs shortly, should I upgrade to 13/16 bore? Thanks-

 

Josh

 

Before throwing money at this problem, adjust E-brake and all four brakes. Cost = $0