Brake Pedal Soft on Slow Application; Solid on Fast Application

[msmith65]

'93 2WD with power brakes, 165,000 miles.

When I apply the brake pedal slowly it goes about 3/4 down before the brakes start to work.

But if I apply the pedal fast, I get a solid pedal and good braking. Same if I apply the pedal twice, but at a medium rate.

I topped off the fluid and checked the back (drum) brakes for adjustment. No improvement. Shoes and pads are good.

Am I looking at a master cylinder?

 

[Iron Weasel]

'93 2WD with power brakes, 165,000 miles.

When I apply the brake pedal slowly it goes about 3/4 down before the brakes start to work.

But if I apply the pedal fast, I get a solid pedal and good braking. Same if I apply the pedal twice, but at a medium rate.

I topped off the fluid and checked the back (drum) brakes for adjustment. No improvement. Shoes and pads are good.

Am I looking at a master cylinder?

Possibly. The fact that it's solid if you pump it twice makes me thing the seal in the master cylinder may have ruptured.

Check around the bottom of the master cylinder and on the brake booster underneath the cylinder and see if it's wet or if the paint is bubbling. Also, you may want to check (if you haven't already) the calipers on the front and the wheel cylinders on the rears for signs of leakage as well.

 

[Anime]

It could very well be a master cylinder, but lots of first gens go 300,000+ miles on the original master cyl, so it's not always just that.

If the pedal goes down 3/4 of the way when pressed normally, it's more than likely the master, and the pressure of a quick engagement is what's making the difference.

I'd still say it's worthwhile to check the rest of the braking system though, especially the front and rear lines for any cracks in the metal, and the hoses as well, for any leaks or bubbles.

If the hoses aren't in the best of shape, or any other part of the brake system looks like it might need replacement, you might as well do it along with the master cyl. I would also suggest all-new brake fluid since you'll be bleeding the system anyway.

If you don't find any other possible causes, I'd suggest replacing the master cyl. ASAP. The cost and the hassle of replacement will be well worth it compared to the safety risk of getting into a collision, and the associated costs of that.

 

[Roadrunner777]

Agree on master cylinder. There are cup seals inside and a faster push is making them seal better against the wall of the cylinder. Are these rebuildable? JC Whitney has rebuild kit for $26 + shipping, Orielly stocks it for $40. I'd go to Orielly

Edit: I just thought about the brake booster and if it might have a bearing on this problem. Not sure how to rule it out. I think I'd do the master cylinder anyway.

 

[msmith65]

Thanks, guys. I swapped out the master cylinder (from O'Reilly; same price as RockAuto) and it works great.

It looks like I did this once before -- left a trace of a shop towel in the cup of the plunger, on the outside. I think it was RockAuto but I can't find the receipt.