fading brake power @ idle speed

[ddm156]

I have a problem with my braking power, most noticeably at idle speed. As I slow down, for instance - a stop sign, I have decent brake pedal to work with, and the truck slows to a stop. As I sit there and putt the truck forward before pulling out, the pedal will fade/sponge the whole way to the floor. The brakes still stop the truck (barely), but the firmness in the pedal is gone. Once I have the truck rolling down the road again, the pedal is ok. Its just when I'm putting around it seems to be soft????? I have the chevy front disc setup, chucks rear caddy brakes. Factory power brakes, with a vacuum booster. Any ideas/experience??

 

[ddm156]

by the way, my booster is the one that was on the truck since I bought it- likely from factory, when it still had drums at all 4 wheels.

 

[robofig]

Possibly the check valve, at the end of the hose that plugs into the booster. Real cheap to buy.

 

[rsharpnm]

Likely a vacuum leak when brakes pressed. The check valve is easy to check out... but if you hear a faint hissing sound when the brakes are all the way down, you've got a leaky power booster, methinks.

Might be time for a replacement booster.

 

[blubuckaroo]

A vacuum booster only assists the brake rod. A problem there shouldn't let the pedal go to the floor. It sounds more like a leaking master cylinder. The ones I've seen fail will work when the brakes are first applied, then let the pedal drop at a stop light.

 

[gearida]

My bet is on the leaking power brake booster.

 

[Apogee]

A vacuum booster only assists the brake rod. A problem there shouldn't let the pedal go to the floor. It sounds more like a leaking master cylinder. The ones I've seen fail will work when the brakes are first applied, then let the pedal drop at a stop light.

X2...although not to say that the booster might not be failing as well. If it were just the booster, the pedal will feel high and firm but the braking effort would go way up. Since the pedal is sinking to the floor, your MC is probably bypassing fluid internally assuming it's not leaking fluid out the back and down the booster...that's a pretty easy check.

Either way, I see a new booster and MC in your near future.

 

[broncnaz]

X3 on the MC If the booster was failing the pedal would get a little stiffer and not go to the floor.

 

[BG's 68 Bronco]

New MC needed!

 

[ddm156]

thanks for the help. with the chebby front discs and chucks rear caddy disc kit, which master/booster setup would be the best, and why?

 

[rsharpnm]

X3 on the MC If the booster was failing the pedal would get a little stiffer and not go to the floor.

Uhhh... can I change my mind, too? You guys are right... I don't have a booster, but I've had a bad MC, and what you're saying makes perfect sense.

Besides, it's easier and cheaper to replace the MC first.

 

[BG's 68 Bronco]

thanks for the help. with the chebby front discs and chucks rear caddy disc kit, which master/booster setup would be the best, and why?

Do a seach, but I believe the corvette disk/disk MC is one of the most desirable. Can't remeber what year though. Late 70's early 80's?

 

[blubuckaroo]

This is the complete replacement booster/master cylinder:

Shucks/Kragen/ORIeilly/Checker.
Cardone Loaded Power Brake Booster
Part Number: 509317
Vehicle: 1976-77 Ford Bronco
Engine: V8 5.0 Liter V8 5.0 Liter Carbureted
Power Brake Booster: Remanufactured; With Master Cylinder
Our Price: $145.99

Getting the complete unit avoids having to adjust the push rod length. Just install it and bleed the system.

 

[ddm156]

If I want to try replacing the master cylinder first, am I able to buy a master cylinder ONLY from the 76-77 DISC early bronco, or will the master cylinder only not be a direct swap into my existing '72 power brake setup for drums??? Does the pushrod size come into play or anything else I'm missing, or can I go into NAPA and buy a master for the 76-77 and bolt right up?
Also, napa sells a booster/ master cylinder setup that is about $150, where the vendors sell real shiny ones for much more. Is the extra money just for shine?? Anybody go this route?

 

[Broncobowsher]

Another for the master cylinder...
Firm braking makes the seals in the master cylinder seal up. Light pressure isn't enough to keep the seals tight to the bore and the pedal drifts toward the floor.