145 Brake Issue

[OldCarNewTricks]

Hello, I am currently helping my friend with his 1969 145. The brakes have been an ongoing project, as when he purchased it, the booster leaked internally causing the idle to rocket every time he was on the pedal. Eventually, the plastic diaphragm decided it had had enough and shattered. We recently replaced it with a new unit from CVR. I made sure to adjust the linkage from the pedal to the booster and set the pedal height as it should be. We also bled/flushed the system at the same time. The weird thing is that now we only get about 1" of pedal travel before the brakes (aggressively) lock up. All four brakes are receiving pressure, no aggressive pulling to one side or the other. The booster IS working, so it's quite an odd one. Anyone experience this before? Wrong master or maybe locked up caliper pistons?

 

[84B23F]

Years ago, I fetched a booster/master-cylinder from a heavier vehicle and placed it in a much lighter vehicle.

Light pedal pressure was all that was required. Most likely here, Vac/Master were the issue.

"a larger Hydrovac (tandem diaphragm) will increase brake pressure compared to a smaller, single-diaphragm unit because it generates more boost, resulting in greater hydraulic pressure at the wheel cylinders for a given pedal input."

 

[dl242gt]

When you adjusted the pushrod into the booster. Was it on the high or low end of the adjustment? I'm thinking you may need to increase the adjustment to account for the brake expansion while driving.

 

[OldCarNewTricks]

Years ago, I fetched a booster/master-cylinder from a heavier vehicle and placed it in a much lighter vehicle.

Light pedal pressure was all that was required. Most likely here, Vac/Master were the issue.

"a larger Hydrovac (tandem diaphragm) will increase brake pressure compared to a smaller, single-diaphragm unit because it generates more boost, resulting in greater hydraulic pressure at the wheel cylinders for a given pedal input."

Larger bore MC would make sense. Not sure, everything appears to be factory spec.

 

[84B23F]

Might try without vacuum....

I would not think distance traveled is relevant, but amount of pedal pressure. Different topic, but when bobtailing a semi-truck the other day, more pedal pressure was required than with a trailer attached.

I would like to think there is a relationship between pedal pressure and braking force when driving on snow for consumer vehicles.

 

[OldCarNewTricks]

I know that the brake control system was redone a couple years ago by the previous owner. Not sure if they also swapped out the MC. No casting numbers to check online for.

I'm certain it's not the booster. It had the same amount of travel when broken, just not power assisted.

 

[dl242gt]

Looks like a aftermarket master cylinder. Maybe different bore size in it?