soft pedal after master cylinder and booster replacement diagnosis?

[spedigo]

I just replaced the master cylinder and the brake booster on my 79C 10. Afterwards I have a soft pedal or you might say it has a long travel before it engages near the bottom. Grab‘s fine at that point but definitely not like it was before.

Yes, I bench bled the new master cylinder and there did not seem to be any air pockets remaining.

I put a centering pin in the proportioning valve before beginning the swapand it did not move as I bled the lines. Did not seem to be any air pockets in any of the lines after bleeding them, either.

I put in a new JB5 power booster and measured the push rod length before I installed it and compared it with the old unit and it was within an eighth of an inch the same measurement.

my first thought is to adjust the pushrod length and make it a bit longer. One turn at a time, and seeing if that makes a difference.

any other thoughts or suggestions would be appreciated

 

[Ricko1966]

I just replaced the master cylinder and the brake booster on my 79C 10. Afterwards I have a soft pedal or you might say it has a long travel before it engages near the bottom. Grab‘s fine at that point but definitely not like it was before.

Yes, I bench bled the new master cylinder and there did not seem to be any air pockets remaining.

I put a centering pin in the proportioning valve before beginning the swapand it did not move as I bled the lines. Did not seem to be any air pockets in any of the lines after bleeding them, either.

I put in a new JB5 power booster and measured the push rod length before I installed it and compared it with the old unit and it was within an eighth of an inch the same measurement.

my first thought is to adjust the pushrod length and make it a bit longer. One turn at a time, and seeing if that makes a difference.

any other thoughts or suggestions would be appreciated

The pushrod length is measured in thousandth of an inch not inches. You want .020 clearance between the pushrod rod and the end of the master cylinder. Did you remove and replace all identical pieces or is this a mismatch of jd3 brakes jb3 brakes jb5 brakes,and something your friend had in his garage? Was this car always Jb5 with power brakes?

 

[spedigo]

The pushrod length is measured in thousandth of an inch not inches. You want .020 clearance between the pushrod rod and the end of the master cylinder. Did you remove and replace all identical pieces or is this a mismatch of jd3 brakes jb3 brakes jb5 brakes,and something your friend had in his garage? Was this car always Jb5 with power brakes?

as far as I know, it’s always been JB5 brakes. At least the power booster I took off was. Assuming that was a Big Ten thing.

Booster and master cylinder are both brand new.

that’s why I was assuming that pushrod length must be off a little bit and that’s where the difference is.
honestly, I was running out of daylight fast, so I just did a quick tape measure measurement to see if the push rod length was the same as the old power booster I was removing.

 

[ali_c20]

What is the bore of the new master cylinder? Is it the same as on the old master cylinder? Smaller bore makes a softer feel and longer pedal travel. Bigger bore hard and less travel travel to sum it up quick and dirty.

 

[spedigo]

yeah, I had read that somewhere before. I didn’t compare to the old ones bore size since I knew the one I was purchasing was the larger size, 1 1/8. I measured it at oreilly before I purchased it to make sure because some of them were definitely smaller. And also to make sure that the pushrod end mated with the piston as it should.

 

[Ricko1966]

You need to check the clearance between the end of the pushrod and the booster. Clearance should be .020. Quick and dirty check put a smal dab of toothpaste on the end of the plunger,push the master onto the booster studs and up against the booster. If it pushes to the booster without resistance or bottoming out the plunger isn't out too far if it transfers toothpaste from the plunger to the master it's close enough. I was taught 30 years before I learned the real measurement. You want a tiny bit of clearance,so it's not exact and not rocket science

 

[spedigo]

You need to check the clearance between the end of the pushrod and the booster. Clearance should be .020. Quick and dirty check put a smal dab of toothpaste on the end of the plunger,push the master onto the booster studs and up against the booster. If it pushes to the booster without resistance or bottoming out the plunger isn't out too far if it transfers toothpaste from the plunger to the master it's close enough. I was taught 30 years before I learned the real measurement. You want a tiny bit of clearance,so it's not exact and not rocket science

cool, thanks.

Yeah, that’s basically what I did this morning before work. I put some thick paste in there, and I tried it with the new push rod and the original push rod from the old booster (neither are adjustable).

there’s a 3/8 difference between the two. The old one is longer. with the old pushrod in it definitely made the pedal stiffer, but too stiff.

at any rate, the paste was helpful and it’ll help with getting me back into the ballpark. once I get there, I’ll just grind the old push rod down to the right lane and go with that.

Thanks everyone.