c5_droptop

Done some searching but couldn't find this situation.

All started a while ago with decreased braking power. To be honest, I don't remember the exact symptoms but given that I didn't know all the history of the car I decided to do some major part replacement.

So to VBP and a new master cylinder (factory), new calipers (stock with SS sleeves), new SS flex lines, new rotors (slotted) and new pads (Hawk). Good news as the fluid was pretty poor and the rest looked a little worn, so still a good idea to do some work.

Any case, been fighting getting a good braking system since then. Some comments on the current situation:

1. Hard brake pedal when driving. Hard problem getting the pedal to the floor under braking. Brakes don't lock up. No appreciable pull to one side or the other.
2. Hard brake pedal when car is off.
3. Using synthetic fluid. Bled multiple times. Don't seem to be getting air. Can get the pedal to the floor under bleeding. Found a bad caliper in one bleeding (leak) with no leaks since being replaced.
4. Hooked the carb directly up to the booster. No difference in behavior.
5. Don't appear to be any leaks in the system.
6. Hard lines are original and there does not appear to be bulges in them or the flex lines.
7. Proportioning valve has not been replaced. There has been no issue with the brake light coming on so I don't believe it has failed (unless it isn't hooked up right -- I haven't checked that).
8. Tried warming up the brakes and there didn't seem to be a difference.
9. Have not checked run out (don't completely understand) but I have had this issue since the minute I put the new components on. Willing to check though.

It's just like there isn't enough clamping power.

Possible thoughts:
1. Blow by in the master - would expect a soft pedal though
2. Synthetic not working well in the system - would also expect a soft pedal if things were agitated or otherwise having issues.
3. Air in the system - would also expect a soft pedal
4. Poor shaft throw in the booster to the master - would expect that I couldn't bottom out the pedal in bleeding
5. Poor brake pad to rotor compatibility - could it just be bad performance?

Interested in feedback as well as a bit of the reasoning behind it.

Thanks,
Dave

SH-60B

The booster is probably leaking vacuum, or the port on the carb is doing nothing. Pull a vacuum on the booster, maybe with a mighty-vac, and see if it holds.

DRIVESHAFT

A hard pedal is most likely to be caused by either a bad brake booster, or a bad vacuum supply to the booster.

jyounane

Hi, I'm not a brake expert but happy to kick off discussion with some thoughts.

Assumptions
1. 69 Corvette PB, no proportioning valve, just brake distribution block / failure switch
2. brakes bled ok so flexible lines not blocked.
3. Correct master cylinder and booster combo

A hard pedal generally indicates little or no air. It should not be possible to push the pedal to the floor unless there's a lot of air or a brake bleeder is opened. (Of course "hard pedal" is a subjective statement)

If you have a properly working brake system with a "hard pedal" but still have poor stopping power, then I'd suggest looking at the pad compounds and perhaps choosing a softer pad. Braking does improve a little when pads and rotors "bed in" but not very much.

If you suspect braking system not working properly... then you'll need to try a few things.

I'd try applying brake pedal pressure while engine is off and then without releasing the pedal, start engine and see if pedal goes down further. That is what should happen if booster is working normally. If it doesn't, you could start investigating booster.

Check that you have adequate vacuum at the booster in any case and that the booster check valve is OK. Is the engine / cam stock?

I don't think the choice of silicone fluid is a factor... but I'm no expert.

Let us know what you find. As you know, there are many here on the forum with the knowledge and experience to help.


Joe

75vetteman

booster was my thinkin too. hard brakes bein that it acts like manual brakes with no assistance from the booster. don't go to autozone for this though. i did this n it didn't work properly. i had to get a period correct one for double the money- $200
have fun installin it! it was a PIMA (pain in my a**) the bolt in the upper left was really hard to get to on mine

45ACP

What exactly did you put on for pads?

If the compound is of the type for track use only they will stop very, very poorly until they are -very- hot. Street use requires street pads.

7T1vette

Possibilities: Bad master cylinder; bad/blocked P-valve; bad vacuum booster (if P/B); mechanical restriction on brake pedal linkage that limits travel.

EmNash

I read somewhere that having the wrong master could cause this type of problem. Cant remember where though....

c5_droptop

Thanks for the feedback thus far...

45ACP/jyounane: The pads are Hawks. Need to verify what particular model. Assuming that VPB wouldn't package race pads for their stock rebuild kit.

jyounane: Quick question the booster test mentioned above ("applying brake pedal pressure while engine is off and then without releasing the pedal, start engine and see if pedal goes down further")... is the thought that with the booster there would be a pressure assist that would allow for more pedal actuation.

jyounane: On the check for the leak by pulling a vacuum to the booster. Just listen for a hiss or is there a better way to try and track a small leak (one friend mentioned possibly misting the area with WD40 and look for it being sucked in).

jyounane: Engine is not stock. PO put in a relatively aggressive street cam bore/stroked to 383. I will check the vacuum at the carb (actually EFI) but the brakes were good for a while with this engine configuration so I am thinking that something else changed.

I will start researching sources for the booster while I wait for some time to get the tests done.

Thanks again one and all...
Dave

jyounane

Dave,

Yes, once engine starts and generates vacuum, the booster will create an assist and pedal will travel a bit further. If it doesn't, it will point to a booster or vacuum issue. So let's start by trying that. (You can also try it in your daily driver to see what the change feels like.)

Let us know what the results of the quick test are.


Joe

c5_droptop

Went out and tried the test....

Foot on the brake. Started the car. Slight movement, but not a huge change.

So, sounds like the booster isn't flat dead.

I will try and Frankenstein something together for the vacuum test this weekend.

Dave

jyounane

OK...

(these things are subjective so it's always difficult to give advice without being there... but if the pedal did move further... then the booster is probably working to some extent)

Next thing you'll need to check is how much vacuum you are getting. A vacuum gauge "Tee'd" into the line to the booster will get you the answer.

Good luck.

Joe

RobbSalzmann

Did you turn the rotors before installing the new pads? Even if you bought new rotors, they need to be turned to break the machining glaze & true up the run out. If you put new pads on old rotors without turning them, the pads will glaze up, not bed in and will have a lot less friction than they normally would. Warming up glazed pads will exacerbate the issue.

Runout is the amount a rotor is warped, and presents as a "wobble" when you look side on at the spinning rotor.

Peterbuilt

True or false?
These people say not to turn a new rotor.
http://www.oeqf.com/techinfo/index

This is about checking run-out
http://www.brakealign.com/

ramair_bryan

Nashvettes got it........Master cyls piston/bores to big. Happened to me too. Takes a bigger foot. I matched it up to the old one and took it back. The smaller piston takes less shoe size. . Love you guys.

1427

Bore size for power brakes 1/1/8th"
Manual brakes 1 "

Ricknhis69

I had this problem for years, assumed it was because I didn't have power brakes. Found out that with a dual circuit master cylinder you can have one circuit go bad and not even know it just from driving the car. Later discovered that somebody had disconnected the brake warning light that is wired to the proportioning valve. (probably assumed that because the car stopped nothing was wrong so it must be the light. Genius) After checking one of several service manuals I have I found out that the hard pedal and insufficient braking is indicative of having a master cylinder with one of the circuits being bad. Makes sense, only one axle is providing full braking. Also, any weakness in one circuit compared to another would mean that the master cylinder is bad, it doesn't take a complete failure to make it bad.

I know you said you had a new master cylinder but more than once I've seen ones that are bad right out of the box. I do believe you should have a proportioning valve and not just a distribution block. If its working properly and the brake light (on the dash) is not on (but then again, who knows, the bulb could be burned out) then your master cylinder is probably ok. The light comes on if the valve is slammed all the way to one side, as would be the case if 1/2 of the master cylinder failed. If I remember correctly, the "brake" light lights when the proportioning valve provides a ground so maybe you can check the light and the proportioning valve based on that info.

If no one else is able to confirm this off the top of their heads I'll dig out my service manual and double check.

In regards to the 1" vs 1 1/8" issue, if you had a manual master cylinder on a PB booster your brakes would be excessively touchy, not hard like you are describing. A hard pedal would be true if you had a PB master cylinder on a manual brake car.

Good luck.

jyounane

Hi Ricknhis69.

I agree in general with most of what you've described.

However, I believe the 69 has a distribution block, not a proportioning valve. It works the same way you describe in the case of a circuit failure and will light the brake warning lamp.

The function of this block, and or proportioning valve has been the subject of some discussion on this forum...

I hope others will confirm my understanding.


Joe

Ricknhis69

Checked the book, yup you're right. They made the switch in '74.

c5_droptop

Finally found the time and patience to remove and replace the booster after verifying at a friend's shop that it was not holding vacuum. What a pain.

Well, now I seem to have great braking and a soft pedal... can lock the brakes up pretty easily but the pedal takes a while to engage and some time to rebound. It does seem to firm up if pumped though.

When we were testing the booster I did notice some interesting items.

I started with a master like the one on the left. The replacement was like the one on the right. Didn't really notice early in the process.


Funny thing was, there was no adjustment on the booster up to this point, which had the following shaft installed:


I would have expected that with the recess in the master that the brakes would have been barely engaging.

In any case, bought a new booster and installed. Supplied with both a short and a long shaft, so I installed the long one. That is where things stand now.

Next steps/questions:
Will try and bleed again.
Heard that there were two differnt Booster and two difference Master options... assumed that since I have seen both masters and tried the longer booster shaft that I have to have the right combo. Anyone know any different?
Had a bear of a time with the cotter pin from the passenger compartment booster end to brake pedal but had a heck of a time seeing if there was any other adjustment possible. Did measure the rough length from the booster face to the booster fork and they both looked very close to one another. Ideas?

Thanks,
Dave