Brakes only slow the car and won't stop on a small incline

[1972avantiisme]

I've had this Avanti for several years.  The previous owner did a body off paint job and a cleaning and freshening up of the frame etc. I rebuilt the front calipers.  There was already a new power brake booster and new master cylinder with two compartments. I just bled the brake again and noticed that the brake fluid in the rear compartment of the master cylinder fed the fluid to the front brakes.  This doesn't seem correct to me.  Is it possible that these are reversed and therefore there is not enough braking to stop the vehicle?

It also seems that the pads on the brakes are dragging as the disks don't spin without some pretty good pressure?

Any help would be appreciated.

By the way, I am also the member of several other automobile organizations, because I have several other collector cars,  but I am very impressed that the Avanti AOAI  organization and their magazine.

[John D]

not sure if it is same as my 79 but mine has a larger reservoir in the rear (closer to the booster). Large reservoir should be to the front brakes. 

Mine was hooked up backwards when I replaced it this spring. The pistons are the same diameter so they provide equal hydraulic power. I studied the parts inside the removed one and read the Stude shop manual for my 64 Commander dual mc,  and it's very similar, in summary if the rear system goes you don't lose much pedal but if the front does, it'll be quite noticeable, as you'll be operating the rear system thru a spring for the first inch plus of travel. 

I looked into it because I didn't realize it was potentially hooked up backwards until I hooked it all up and bled the fronts and found the smaller reservoir had gone down. I really wanted to leave everything as is but eventually convinced myself to re-plumb it. 

Backwards hook up would not result in reduced braking with a good system, the difference is only with a failure. 

I replaced the MC because the power booster had failed, the car stopped ok but not well when I bought it. So double check your booster... be sure it's actually boosting! 

[1972avantiisme]

That's right front brakes rear reservoir.  I was told all GM master cylinders are that way.

[Mel]

There is a You Tube video on checking the operation of the booster -- "BRAKE BOOSTER PERFORMANCE CHECK".  He isolates the booster from the system, he actually holds it in his hands at the side of the car, and runs it through the checks.

If the brakes are dragging, you may have the adjustment rod in the end of the booster a bit tight up against the m/c piston which will keep pressure on the piston and, hence, pressure on the pads.

Good luck.

Edited September 30, 2022 by Mel

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Are the caliper pistons installed properly in the cylinder bores?

Do the pads fit properly without binding in the caliper? (I've had some pad sets that were slightly too large and had to be sanded down on the sides to fit properly)

Are the brake calipers properly shimmed so they are centered on the rotor?

It's normal for the caliper to drag on a bit after the brakes are released but it should be fairly free after a few revolutions of the wheel. 

As mentioned above, if the booster pushrod is out of adjustment this will also cause the brakes to drag. If you feel that the brakes are dragging, one way to check this is to loosen the nuts between the booster and the master cylinder to separate them.  (with the car on jack stands, do not drive it like this) If the dragging goes away the pushrod is too long and needs to be adjusted. 

[Mel]

Update on my note above.  I'd replaced the booster and master cylinder and thought I had everything adjusted properly but, apparently not.  The braking had a very hard pedal and acted as if the booster wasn't working at all.  I performed the check on the booster as referenced in my note above; it was working properly.  I placed washers between the booster and m/c for spacers and everything worked properly -- i.e., the rod was adjusted a bit long.  Interestingly, the pads were not dragging.  Hope this helps.