Brakes work great after high RPM ? ? ?

[jfabert]

I have been fighting my brakes for the last ten years. I have replaced everything except the hard lines and the booster. no air or blocks in the system. New engine with plenty of vacuum.

When braking the front end drops a lot and it feels like the rear applying very slowley. The other day i was driving up a hill on the highway thats about 3/4 mile long, rpms around 2500 and at the stop light on top of the hill the brakes felt amazing. The pedal was very firm and the truck stopped perfect. He front end not dropping like only the front was working. It stopped like my wifes new car. It will brake like that about two more times then go back to feeling like crap. Every time i come up that hill at higher RPMs it will do this.

I figured it would be the other way around being that less vacuum at higher RPMs. The booster doesnt appear to be leaking but should I replace it and the check valve on the vacuum line to see? Thanks in advance for the help.

1987 K5 new 350, 700R4, 208, 4.11 gears, 4" with 35x12.50x15.

 

[AR_K5]

Is the pedal high and hard to push, but ineffective?
Or is the pedal soft and spongy?

If you hold your foot on the brake pedal and start the engine can you feel any change to the pedal on startup?

If it's always high and hard, and doesn't change when you start the engine with your foot on the brake, it's the booster. A booster can also lose partial vacuum. To test for that after a short drive, shut the engine off and wait about a minute or so. You should feel the booster working for at least two pumps of the pedal before all residual vacuum is lost.

If it's spongy, seems to work on start up, and the booster isn't bleeding off, the it's likely air trapped in the lines, or something else.

From what you described, I'm thinking it's the booster or vacuum leak.

 

[jfabert]

When started the pedal moves down a little bit. Also it hold enough vacuum after it sat turned off for one press of the pedal. With the engine running the pedal goes about half way down but feels firm. Shut off it stays high and firm.

I can hear air from under the dash when the pedal is pressed. I thought this is normal because it has done this from the day I bought the truck.

I did notice just now the new master cylinder I installed a couple months ago is leaking at the rear at the piston.

 

[AR_K5]

1: When you installed the new MC, did you make sure you bled all of the air out of it before you connected the lines and started bleeding the lines at the caliper?

2: Did the previous MC dump fluid into the booster?

3: The whoosh of air you hear when pressing the brake pedal, does it continue if you hold the brakes?

4: Have you inspected the vacuum line and the check valve going into the booster to make sure there are no leaks? With the engine running, try spraying carb cleaner on them and listen to the engine's idle to change. Any change indicates a leak.

5: If you are running drums, have you tried adjusting them?

 

[jfabert]

1: When you installed the new MC, did you make sure you bled all of the air out of it before you connected the lines and started bleeding the lines at the caliper?

2: Did the previous MC dump fluid into the booster?

3: The whoosh of air you hear when pressing the brake pedal, does it continue if you hold the brakes?

4: Have you inspected the vacuum line and the check valve going into the booster to make sure there are no leaks?

1) Yes i did. I took my time and bench bleed it using a kit with the hoses that circulate the fluid back into the reservoir.

2) The last MC did not lead but it did allow the pedal to slowly fall when pressing on it.

3) It only makes the sound when the pedal is moving.

4) That was gonna be my next step. I was going to just get all new hoses, filter, check valve and the seal.

 

[jfabert]

I also replaced the drums and shoes when I installed the new MC. The old drums were getting thin and not making very much contact with the shoes, only about an inch in the center of the shoe. I double checked that the new primary and secondary shoes were installed in the correct place and they are the right shoes.

I have not been able to check for the leaks due to me getting sick as hell but as soon as I get feeling better I will get that done and get the MC swapped out under warranty. It has left a spot under the rig now sitting the last couple of days.

The trucks brakes have been driving me crazy for ten years now and I had given up until they worked great after going up that hill. That brought hope back that I wouldn’t have to have one foot on the brake and one on the emergency brake while going down stuff while wheeling.

 

[AR_K5]

Obvioudly if your MC. Is leaking you are going to have to start there.
If everything checks out with the booster, the caliper slides, pads and rotors are good, the shoes/drums are good and properly adjusted, no air in the lines, the next step would be checking the rubber lines for colapse or blockage, and making sure none of the hard lines have been kinked or smashed.

As I'm sure you already know, the only thing about your brakes that is RPM related is your booster.

Or maybe your brakes just worked better due to gravity because you were still on a slight incline.

 

[AR_K5]

Have their been any modifications to your engine that would effect it's vacuum?

 

[jfabert]

The engine is a new Goodwrench crate the same as the original. It idles at 18 -19 inches of vacuum. I replaced the rubber lines with a steel braided set from ORD.

With this MC leaking at the booster could the fluid damage the booster? I may just replace it when i do the MC again. That is about the only thing under the hood that isnt new. I spent the last year and a half tearing it down and building it back up again.

 

[diesel4me]

Yes,the brake fluid can swell up or soften the rubber diaphram--I had to put another P/B booster in my 72 K5 when its master cylinder leaked out of the rear seal and filled the booster up--eventually the pedal became harder to depress,it was so full of fluid (I kept topping it off every few days) and when I started seeing white smoke coming out of the exhaust,and panicked,thinking it was a head gasket that failed--further investigation revealed the engine was sucking brake fluid from the booster,into the intake ,thru the vacuum hose to the booster..
I pulled the check valve out of the booster,and brake fluid came out of the hole!..when I replaced the booster with a good used one,I immediately noticed a great improvement in the braking ability..so much so I almost put myself thru the windshield till I got used to how "touchy" the pedal was now!..

 

[gonefishin]

I was having issues with my rear brakes not working properly. so I accidentally kinked the rear line when working on the exhaust so I replaced the whole rear line and wow what a difference. the back brakes worked perfectly. I think it had something partially blocked somewhere.

besides that I would say you need to replace your booster and check the main vacuum line going to the booster.