Brake Master Cylinder Question...

[PWagon]

I have an '86 K5 Blazer, and it seems like my brake pedal has lost its stopping power. My son will be getting his driver's license soon, and I won't let him drive this Blazer until the braking improves. I've replaced the rear shoes, front rotors, calipers, pads, and upgraded to braided stainless steel brake lines, yet my braking still sucks. I'm not sure, but I'm guessing my 32 year old master cylinder or brake booster may be the culprit. I've never had one wear out, nor have I replaced one. I also do not know what signs to look for to confirm it's bad. I sure could use some suggestions here. Here are a few questions I have.

1. What are the symptoms/signs of a worn out master cylinder (it does not appear to leak)?
2. What are the signs of a brake booster going out?
3. The master cylinders for my year/make are offered in 1.0" and 1.125". How do I tell which one I have?
4. How long does it typically take to swap one out?
5. Are all new/remanufactured master cylinders created equal, or are there some out there with more stopping power built in (for towing and such)?
6. Besides the master cylinder, can anyone suggest any other brake part I might be overlooking as my problem?

 

[Bent77]

6.

You changed out the brakes lines? If the master was working before, you may still have air in the line somewhere

 

[Fordum]

First, and most important, what is wrong with the stopping? When you hit the brake pedal, does the pedal go down and the truck does not stop, or is it just hard as a rock and the truck keeps rolling?
If its hard, turn the truck off, press the pedal down hard a few times with it not running and completely release it each time.
Then press it hard down and hold it while you crank the truck.
The pedal should go down slightly. If it doesn't, then your booster is not doing its job. Trouble could be the booster or the vacuum going to it, but the booster is not working.

If the pedal does go down, the booster should be good.
Next, press the pedal down hard and hold it. It should go down, hit a hard spot above the floor and stay there. If it creeps down, odds are your master cylinder is bad If it feels spongy, in other words, there is no hard spot, you can push harder and softer and it feels springy.
If so, then you probably have air in the system.
NOTE: there should be a slight give in the pedal when it stops, not like stepping on an anvil, more like a thin rug. If its like stepping on sponge rubber, then most likely air.

If so, since you replaced the calipers, you may have put them on the wrong wheels. The bleeders must be at the highest part of the calipers. Many GM calipers are manufactured as left and right, but will fit on either side.
The difference is where the bleeder is.

As for which cylinder to use, did the truck ever have good brakes? If so, and no modifications have been done since, then you want to go back with the same size you had. You only have to worry about the cylinder size when you have changed the basic setup. Such as converting from drum to disk for instance.

If the truck had good brakes, when did they go bad? Did they go bad and then you did all the replacement? Or did you do the replacement to make the truck more safe for your son and the brakes came out bad?

Inquiring minds want to know......

 

[diesel4me]

Define "braking sucks"...
Do you notice the pedal comes up higher and feels firmer after pumping the pedal a few times ?..if so,there is air still in the system,it needs to be bled..

If the pedal sinks slowly to the floor with very light pressure on it,then the seals in the master cylinder are worn and leaking internally.and it needs to be replaced..

If the pedal feels hard,and you have to really "stand on it" to stop,then I'd suspect the vacuum booster is not functioning,perhaps its not getting enough vacuum,check for leaking or cracked or collapsed vacuum hose, and the plastic check valve at the booster--the power booster develop cracks and holes in the rubber diaphragm that provides the power assist after many years..often you'll notice a rough idle when the brakes are applied and you may hear hissing under the dash,that is the engine vacuum drawing air in through the failed diaphragm...

To determine which master cylinder size your truck had,you can either measure the piston on the master cylinder once it is removed--or you may find a casting number on the casting that can be cross referenced to the correct one listed for your vehicle..

Be aware someone else may have swapped one on off something else or got the wrong one years ago,being so old...the larger piston version may make the pedal effort feel harder if your truck was supposed to have the smaller piston unit..I'm unsure if there is a way to determine which one came factory on it--maybe if you have the "build sheet" or options list,or maybe GM can tell by the VIN number..

Usually the larger master cylinder is used when the vehicle has the larger optional brake shoes and wheel cylinder pistons in the rear..
(As in most vehicles optioned with a towing package or heavy duty brakes & suspension..)...

Replacing the master cylinder may take 20 minutes or two hours,depending on whether the lines unscrew from it without twisting up and or breaking off..its only 2 nuts holding it on,you must bench bleed the master first before connecting the lines and doing any other bleeding or you'll just pump air in all the lines again..


I see Fordum just posted before I finished typing--his suggestions are good too,and his way to test the power booster is the factory method..

 

[PWagon]

Here's more back-story:
I've owned the truck for about 10 years now, so I don't know what the previous owners have done to it. The master cylinder and brake booster appear to be factory original. Since I've owned the truck, the brakes have never been great (fair at best). What do I mean you ask? The pedal feels soft like it goes too close to the floor and the truck never "sits down and stops". It feel like my braking just takes too much distance to come to a full stop. A year of so ago I installed Wilwood calipers, and I made sure the bleeders on top. The rotors were replaced with Baer rotors (slotted and vented) since the originals were warped. I replaced the rubber brake lines since they were breaking down and had "lumps" along the hose were they were about to rupture. Yes, I did purge all the air out of the lines (100% confident). The only remaining item I have not replaced would be the booster & master cylinder. They are not cheap, so I don't want to replace them if I don't have to.

 

[Truckman4life]

Replace the master. A worn out one will give you a soft/low pedal and they will just feel like they suck. Also rear drums that are not adjusted tight enough will feel similar. The larger piston size in the master will move more fluid than the smaller one. If you have 3/4 ton brakes then you have the larger setup, half ton brakes get the smaller one I believe. I upgraded my K5 to rear discs and a master off a P30 medium duty step van(even larger piston bore size). It's probably too much for some people since it makes the pedal high and stiff and locks the brakes up with less pedal travel than a smaller bore master cylinder but I like a good firm brake pedal.

 

[PWagon]

@Truckman4life This may be a dumb question, but was there a huge difference in stopping after your rear disk swap? Did you buy a kit? I'm guessing a rear disk setup would use the standard master cylinder (not a special one)? I'll likely swap out my master cylinder, and I've considered doing the rear disk swap for a while now.

 

[Truckman4life]

I'm running an 8 lug ten bolt up front with 3/4 ton calipers and a 14 bolt full floater out back with Ruff Stuff brackets and 1/2 ton front calipers which have a slightly smaller piston than my 3/4 ton front calipers. I have hydroboost since mine is a diesel. I am running the master for an '83 P30 with 4 wheel discs and a diesel/hydroboost setup so it bolted right on. I did an aftermarket disc/disc combination valve and still had too much rear braking. It would try to pass the front in a panic stop or on wet or snowy roads. I put a jegs adjustable prop vslve in my rear circuit and had to turn it almost all the way closed and it's still kinda sketchy on slick roads if you are not careful. For offroad it has awesome stopping power and I love it. If you still have half ton axles there is a guy on here that was doing stuff with front hummer H3 calipers and slip on rotors. Honestly I would probably keep the rear drums on a half ton as they aren't as spendy as a 14bff for parts and when set up correctly they work good. You could always swap to a newer rear axle also with factory discs and internal parking brakes. Oh and yes rear disc master cylinder will be different than rear drum master since discs require more fluid. Lots if guys just soap on rear discs and say everything works great but I don't see how it could with how the system is designed. Rear drums usually have a residual pressure valve built in somewhere too that keeps like 2 psi on the rear circuit for faster operation. That can't be there with discs or they will always drag and heat up.