Mechanics

MODELS BRAKES REMAINING APPLIED

1968 Oldsmobile Other

Brakes problem
1968 Other Oldsmobile Models V8 Two Wheel Drive Automatic 132k miles

Brake pedal required pumping to apply brakes. Replaced master cylinder. After replacing, brakes apply normally for a while then pedal travel decreases on application and brakes remain applied. Pedal hard, very little travel. Thought rebuilt master cylinder was defective. Replaced with another rebuilt unit. Same problem. Note: pedal travel has always been close to the floor when applied. However, condition now experienced is unlike any previously encountered. I'm at a loss. Suggestions on how to trouble shoot would be welcome.
AD
Avatar
Bob Gillum
May 17, 2009.




So let me see if I have this right. The pedal falls but the brakes hang on even though there is little to no pedal pressure?I want to be sure I understand correctly. Does this car have power brakes? Are they disc/drum or drum/drum?

Tiny
Jack42
May 18, 2009.
Hey Jack, thanks for responding. The car is a '68 Olds Delmont 88, power drum brakes. The pedal travel has always been long, almost to the floor, yet stopped reliably. The master cylinder began to fail, in that pumping was required to effect a stop, and while stopped, pedal would drop, and the car would creep forward. Replaced m/c. Thereafter, pedal travel would slowly decrease while driving, becoming high and hard. Brakes would remain applied to the point of increased throttle pressure required to move forward. I'm wondering if the brake hoses may have deteriorated to the point of restricting the brake springs ability to force the fluid out of the wheel cylinders to return to the master, thereby resulting in brakes remaining engaged. Any ideas?

Tiny
Bob Gillum
May 18, 2009.
Sounds like you nailed it with the hoses.I would start there, they do exactly what you describe by holding fluid pressure in the calipers.

Tiny
Jack42
May 18, 2009.
Thanks Jack. Same would apply to four wheel drum brakes, I'm guessing. Possibly just the rear hose, although both front hoses should be replaced, as well. Just to preclude further problems. Hope they're still available!

Tiny
Bob Gillum
May 18, 2009.
Im sure they are, that is just a GM chassis, many cars besides the exact model you have

Tiny
Jack42
May 18, 2009.
Jack, thanks for replying. The hoses, two front and one rear, were available through special order. Replaced all three; problem unchanged. Any additional thoughts on troubleshooting would be appreciated. I'm fresh out of ideas.

Tiny
Bob Gillum
May 28, 2009.
Do you have the old master to compare the depth of the cylinder where the rod goes? The master change resulted in the brakes doing this. The first one just sounds like it was by-passing internally
AD

Tiny
Jack42
May 28, 2009.
Jack, I closely examined the rebuilt unit against the m/c I was replacing. The bore depth of the piston was identical, as was the bore diameter and the line fitting sizes. I did not make any adjustment to the pushrod, although it is adjustable. Is there any possibility that the booster unit itself is at fault, and do you have any ideas on how to troubleshoot possible booster failure? I 'm really puzzled, as I've replaced many master cylinders, and never seen this condition before. Thanks.

Tiny
Bob Gillum
May 29, 2009.
If the pedal is hard to push and very high then it could very well be a booster issue. That is what determines the pedal feel as far as assist goes

Tiny
Jack42
May 30, 2009.
Jack, again, thanks for your timely reply. When cold and initiallly pressed, the pedal travel seems normal. With each application thereafter, the travel lessens, and the brake pedal becomes increasingly higher and harder. Ultimately, throttle pressure is required in order to overcome the brake drag. Is there a procedure to absolutely determine if the booster is malfunctioning? Or would my only recourse be to change it out, and go from there? I'd hate to go to the expense in terms of purchasing a rebuilt booster, and installing it myself, only to have the problem persist. Any troubleshooting tips you're aware of? Thanks again.

Tiny
Bob Gillum
May 30, 2009.

AD