The brake pedal on my '68 Olds Delmont 88 has always had quite a bit of travel, but always stopped before contacting the floorboard. Even so, the brakes applied and released consistently. Recently, the pedal began to sink to the floor when pushed, and required pumping to maintain brake application. I determined the master cylinder was failing, so replaced it. The brakes then worked as before, low pedal travel, but otherwise normal. However, after driving for 5 miles or so, the brake pedal travel decreases, and the pedal feels hard. The brakes remain engaged, as if the brake pedal were being depressed. I suspected a defective master cylinder was the cause, so changed it out. Same problem. The car has power-assisted drum brakes. Any suggestions as to troubleshooting this new, and perplexing, condition? I’m baffled!
It is time to look at the calipers for the brakes remaining engaged. Low Pedal Travel is probably due to air in the lines. Check the vacuum assist to the booster for the hard braking. If you feel unsure of anything let the pro’s take a look, better to spend a few dollars and save a life, yours could be the life you save!
I’m going to give this a shot in the dark here, from what I experienced on my 59 T-Bird. First I did not have a master cylinder problem but noticed that the brakes were getting progressivly worse even after new drums & shoes. ( I installed the new drums and shoes ).
The pedal would go low to the floor but stop ok, then after a few minutes would get hard and lose some braking.
I found out that the rear brakes were dragging as the drums were getting hot.
Turns out the rubber brake line hose that goes from the steel line to both
rear wheels was bad. It would allow fluid into the wheel cylinders but would
not release causing the rears to drag.
One way to find out which wheel(s) are the culprit is when it acts up, loosen a bleeder valve starting with one rear wheel and see if fluid shoots out. if it does is means you have a blockage.
Normally when loosening a bleeder valve you may only get a drop or 2 out if no pressure built up unless you pump the pedal.
When you do each wheel have someone pump the pedal and see if you get a good shot of brake fluid.
I ended up having all the brake lines and hoses replaced and problem fixed.
I took the car to a brake shop where they had to custom make new brake lines as I wanted all new lines & hoses, plus was easier for someone else to do it on a lift than under the car on your back, plus safety came first knowing that everything was new on a 50 year old car.
Hey '59 Tbird, thanks for your reply. Your experience may be relevant to what I’m encountering. Although the problem didn’t occur until after I’d replaced the master cylinder (during which process I bench-bled both rebuilts prior to installation), I did notice a less than strong stream of brake fluid from the bleeder screws while bleeding each of the individual wheel cylinders; i.e., “spurted” rather than “squirted”. I think the steel lines are probably OK, but the 40+ year old brake hoses themselves may be deteriorating internally, resulting in a narrowed fluid passageway. I’ll crack the bleeder screws and observe the results. You may be onto something. Thanks.
Sorry i skimmed through your question (duh no calpers) The rubber on the inside of the rubber hoses, can deteriorate put shreds into the system and cause problems. Still thinking there is some air in the lines for the mushiness and check the vacuum assist for stiff brakes as stated previously.
Waterboy, thanks for responding. I’ve verified proper operation of the brake booster and check-valve assembly, both OK. No air in system, pedal not spongy, just gets high and harder with each application. As you and '59 Tbird have indicated, internal deterioration of one or more brake hoses may be the cause. Thanks. I’ll check it out.
Recently experienced the same brake hose problem on my '76 El Camino. Probably original hoses. The interior lining degrades and acts as a one-way valve. The brakes apply, but do not release.
I’d sure suspect the brake hoses as they do get tired and fail by refusing to let brake fluid flow away from the brake cylinder when the pedal is released. But if it is all four brakes, that’d be a remarkable coincidence – unless there is a hose that is common to all four brakes. And it isn’t quite clear why the brake pedal travel would decrease.
Can you tell for sure which brakes are sticking? The brakes that are locking on will probably be a lot hotter than those (if any) that aren’t. But maybe there is an easier way to tell that is less likely to leave you with blistered fingertips?
If all the brakes that are sticking are serviced by one hose, replace it. Otherwise, maybe the power assist?
How old are the brake springs??? Might be worth getting a brake hardware kit.
A vehicle this old…it might be worth doing a complete brake overhaul (Shoes, brake kit, Wheel cylinders, Lines, E-Brake cable).
Another option is to convert to Disc Brakes. It won’t be stock if you’re trying to keep it stock…but IMHO…it’s one of those upgrades I think is worth it…that along with Electronic Ignition.
The next time this happens loosen the nuts holding the master cylinder to the brake booster (leave the lines attached). If the master cylinder moves forward and the brakes release, the push rod coming from the brake booster is too long. Adjust the to the correct length. Also make sure that the master cylinder is the correct one and that there is clearance between the push rod and the MC piston when released. If you can see the compensation ports, you should get little spurts of fluid into the reservoir when the pedal is first pushed.
It is also possible that the brake pedal and/or the brake booster is not returning to the correct rest position so check the pedal to booster push rod for correct clearance. Also disconnect the vacuum hose from the booster, relieve the vacuum storage, and see if the master cylinder push rod retracts.
Hope that helps. Get back to us when you solve this problem.