Technical Brake pedal problem

Hi all,

I need others brains on this one.

Im working on the brake system on a truck i just hauled home. Manual brakes. Drums all around.

Replaced master cylinder with universal one of the same bore. Rebuilt all wheel cylinders. Bled everything numerous times. Currently have all the shoes adjusted all the way out and locked to the drums.

I can not get a hard pedal unless pumping the hell out of it.

Need insight.

 

First thing we always verify - got free play on the pedal ?

 

Bench bleed M/C?

 

No freeplay in pedal

 

Bench bled multiple times

 

Can't remember exactly but something like 1/8" freeplay between push rod and M/C piston is needed.

 

Should also mention no visible leaks anywhere

 

Where is the master cylinder located and does it have residual pressure valves?

 

Firewall. Single line coming out going to a T, line going forward to a T then across the front. Line going to the rear to a T and across. No valves

 

Any lines that loop up higher than the bleeders?

 

Year, make and model?

 

1960 datsun plg221

 

I think this is a pushrod length problem

 

Nice ! But I think your old master had a residual valve built into it . It prevents the fluid from entering the master cyl . Drum brakes and disc required different lbs ratings one is 2 lbs the other is 10 lbs . Usually a master on the firewall would negate the use BUT I went crazy with a similar situation. Ended up solving it with a $20 residual valve .

 

Could be if the M/C piston is not moving all the way back when pedal is released.

 

What if its not long enough?

 

If your rod isn’t long enough, you’ll never get a good result.

 

I bought a brand new master for a project, could not get a pedal. took it all apart and it was missing a seal on the inside......

 

Very cool little truck!

 

The master cylinder may be bad. For a while I got 4 bad ones in a row, all were new

 

Datsun sure made some funky looking pick-ups back then. The only one of these I've ever seen belonged to a waitress at a local restaurant. She named it "Arlo" and it was pretty well used up. The damned thing had a crank hole at the bottom of the grille and a one-way socket on the end of the crank so you could roll the engine over by hand.

 

As alchemy says, thats no bueno too. Bore and stroke determine the displaced volume of fluid. You may have to measure and compare the stroke distance to the one that came out. This assumes the one you are replacing was right to start with. Also look at the push rod. Is it original or modified? Maybe you inherited someone else's bush fix.

 

I don't think its the master cylinder. I pulled the line that comes off and plugged it. Immediately the pedal was rock hard. Could not move it at all.

Here is a side by side of what i did.



I did have to modify the pushrod to mate it to the pedal. I made it the exact same size at the original one. This is probably where i messed up. There is no adjustment in the pushrod, they used shims on the master cylinder from the factory. Tomorrow's task is digging deeper into this..

I don't want to clutter general discussion with an off topic vehicle. If anyone is interested i made a thread in off topic.

https://www.jalopyjournal.com/forum/threads/rotten-datsun.1335755/

 

That kind of stuff should not happen but, unfortunately it does, putting you behind the 8 ball! I wonder if any brake part companies test their assembled components before they shove them in the box.

 

It doesn't matter what the vehicle or what brakes it has on it you HAVE TO HAVE at least 1/8 of an inch of free play in the pedal when you reach in with your hand and push down on the pedal.




If that port between the piston and the reservoir is blocked you won't have fluid moving in either direction when you need it to.

Also and very important, if you are not using a pressure bleeder and have a helper pumping the brakes it is imperative that you have them PUMP SLOWLY and HOLD the pedal until you tell them to release it or pump it again. My 45 year daughter has helped me bleed brakes since she was 4 and she is by far the best helper at that I have ever had including probably 30 different shop employees or mechanics when I was doing brake jobs daily for a living. A helper who will not follow directions or a guy bleeding the brakes who does not give specific directions on how it has to be done cam make a mess of things real quick. Plus there are a lot of wrong ways and damned few correct ways.

 

Back in the garage. A couple things I'm trying today. When i modified my pushrod i made it the same length as the original, and used the compression spring on it that the new master cylinder came with. As it turns out that spring was compressing all the way before the piston bottomed out. I wasnt getting the last 1/4" +/- of stroke. I removed that spring completely, the original one never had one. Springs on the pedal.

I also removed some of the factory master cylinder shims.

As far as i know i now have full stroke with the pedal now, and a little bit of play. Waiting on my old man to get here so we can try bleeding this thing manually. I've lost trust in my pressure bleeder.

 

Have you considered gravity bleeding it? Add drain hoses to the bleed screws and leave em cracked open. Do NOT hit the pedal just let ‘em weep. The weight of the fluid in the master will keep ‘em dripping a bit. Check the reservoir every 20 mins or so, and clean the garage, have a beer, or whatever. I have also rigged up an auxiliary reservoir and left em over night. Saves a lot of pumping on a new system.

 

Great idea!

Those 2 changes i did made a massive difference. Its still not perfect though. Gotta bleed more i guess