'98 master cylinder "spurts"

**MrShorty** · 11-29-2008, 05:45 PM

R/R'ed front brakes today. Everything went well, but one observation has me concerned about the master cylinder.

After getting everything back together, I got set up to bleed the brakes (at least, the calipers and wheel cylinders, I don't have the computer stuff to bleed the ABS system). I took the cover off of the master cylinder reservoir, and, when I pressed the brake pedal, brake fluid spurted out the top of the reservoir. I put the cover back on, and proceeded to bleed the brakes without other incident, but it bothered me, as I haven't seen that behavior from other brake systems I've done.

I expect someone will ask: the pedal doesn't go to the floor, and I can stop. The pedal is spongier than a lot of vehicles I've driven, but I've seen many discussions that suggest GM brakes are just softer than others, so I've chalked it up to normal pedal feel.

So my question: Is it normal for the master cylinder to spurt like that with the cover off when the brakes are pressed? Or does it indicate that a seal isn't sealing right?

**tbplus10** · 11-29-2008, 07:10 PM

It happens sometimes. As long as you didnt run the resevoir dry or suck in air your good.

**ct9a** · 11-29-2008, 09:20 PM

I did the same thing. I'm pretty sure that it's normal, I put in a brand new master cylinder and it happened to me after that.

**MrShorty** · 11-29-2008, 11:22 PM

Thanks, guys. I won't worry about it for now.

**KirkW** · 04-05-2009, 09:45 AM

It's normal - the master cylinder on our brakes has a "Quick Take-Up" valve. The front calipers are 'low-drag' calipers that pull the pistons back about twice as far as regular calipers. This reduces drag and increases fuel mileage (slightly...).

However, a regular master cylinder doesn't supply enough fluid to push these pistons back out far enough before the pedal runs out of travel. So our master cylinder has a second, larger piston for the front brakes. This supplies a large volume of brake fluid to get the pistons to move out. Once the pistons have made contact the pressure in the system rises. That's when the "Quick Take-Up" valve opens and the extra fluid is vented into the reservoir in a geyser-like rush. All this happens in the first 1/4" of travel.

You can read much more about it here (and how to diagnose problems with C/K series brakes):

Brake & Front End Magazine: Follow a Diagnostic Process When Curing a Low Brake Pedal