Yesterday morning I backed the car out of the shop and didn't think it was going to stop after backing down the sloped ranp because the pedal was so close to the floor. Took it for a drive and the pedal height quickly increased to the same 1/2" from the floor as yesterday. The car stopped better than it did yesterday. In the evening I took it for a 20 minute drive and before I came home the pedal height increased to about 3'4" and the pads seemed to be well seated as the brakes really clamped and the car stoped well. It seems the pedal height is close to where it should be if it stays there. I don't know why the pedal height is increasing as I drive but Im sure happy that it is. The shop owner is supposed to come by after work today and we are going to pull the booster off and lenghthen the pedal to booster push rod as per Easy Eds post if we can. I think I will leave the shop owners new master on the car instead of trying another junk chrome one. It seems my problem was bad masters the whole time although there may be something else going on if I have to increase the rod length.

 

I just took the car for another test drive and when I backed the car out of the shop I had almost no pedal, same as yesterday morning. After driveing for a little while and applying the brakes the pedal height increased dramatically. By the time I got home I only had to barely depress the pedal to feel the brakes applying. Perfect !!! I don't think I need to or could adjust the pedal rod length at this point because I believe it would cause the brakes to drag. I'm not sure why I have almost no pedal when I first start driving and quickly wind up with a good pedal. Maybe we will just bleed the system again tonight. I might still have some air in the system causing the initial low pedal. What do you guys think. Noonie .... Rodger?

 

I can't remember ever having one act like yours,I'm still leaning towards air in the system.

Bad boosters- In my experience a bad booster usually just gives a hard pedal with no assistance,like running one out of vacuum will do. I've had boosters hiss excessively while applying the brake but would still give assist. When the rod is too short , if I remember right,the pedal goes down quickly before contacting the piston in the master.

Most cars I work on the brakes is after I've done a brake job but I have done bleed jobs only and have run into some pretty weird stuff happening.

 

I've also had masters that will pump up but will bleed clear to the floor with light pedal while sitting at a stop light for a long time.

 

Billy,
Sounds like you are having some success at last, or progress at least. Is it possible that the vacuum in the booster is building up over time as you drive and apply throttle? That may not happen at idle in the garage, but could happen at higher revs as you drive. What is your vacuum pressure at idle and higher revs? I'm just guessing here, as usual. Maybe your shop guy has a vacuum gauge he could bring along if he is coming by this PM.
Ed

 

If I remember right, we had 14 or 15 inchs of vacuum when we put the new engine in. I've messed with the timing a lot so I'm not sure if that has changed the vacuum or not. I could check it with my vavuum gauge.

 

That's really subjective, everyone's idea is different. How far from the top (released position) 1/4 way down, 1/2 way down 3" ???
BTW, bedding the pads always helps, but not so much with pedal height. It just grabs better sooner, so you don't have to press so hard. Is that what you're feeling?

I thought you looked at your car and the pic I posted and also took the pin out and determined that the pedal couldn't go up higher.
The people that had booster rod to pedal problems, unless I misunderstood, had the wrong booster/rod and could make up around an inch at the rod and pedal.

Those old boosters are really pretty powerful and if you pay close attention the brakes will start almost immediately like you found.
I really doubt you have had 4 masters bad, but you never know.

I've only had a couple over all the years, with the worst one having the pedal go to the floor all by itself by just starting the engine and not touching the brakes at all. Brakes were fully applied at the wheels and master was good. Should have put a pressure gauge on, just for fun.

Yes, that usually happens when the seals just start to go, but when pressing hard, they work just fine.

I'm afraid I'm still with the booster malfunctioning internally, especially after all these masters.
I suppose you could try a known good master from another car and see if it acts the same way.
If you can find a route safe enough, perhaps try driving without vacuum to the booster to eliminate it. The pedal will be very hard, but if it stays near the top, then it would eliminate the booster.
As far as your vacuum level, the lower the vacuum, the harder and higher the pedal.

 

I heard you say that earlier in this thread,that's really weird and a new one on me.


Exactly,as long as you hit it hard you have brakes.

But if the master is bled good,shouldn't the failsafe(rod inside)takeover and the brakes would just revert to manual ?

Sure wish I didn't have so much to do to get ready to go south,Id take a road trip and find out WTF is going on.

 

Billy,
I don't know if the vacuum pressure buildup has anything to do with the improved brake performance. I'm just spit-balling the results you are having when you drive for a bit. What changes to make the brakes wake up?
My understanding of the vacuum system is this:
1. Car off, with the vacuum bled off by depressing the pedal a couple times, atmospheric pressure on both sides if the booster diaphragm(s).
2. Car on, vacuum on both sides of the diaphragm until the pedal is depressed. At that time atmospheric pressure enters the pedal side while there is still vacuum on the master side of the diaphragm giving the assist. (Could be wrong so correct me if I am, anyone, please)

Maybe the vacuum on the pedal side is not released until you hit the brakes a couple times as you would when driving? (Don't think so, but you could try that before setting off.) What else changes???? Friction, vibration, fluid compression, what?
Ed

Better yet what changes to get the results you are seeing?

 

Warped pads,cocked pistons,partially blocked brake hoses,calipers full of gunk might cause the pedal to get better as you drive. Can't remember what a master to booster rod being too long does,but it does cause some weirdness.
I wonder if o ring pistons with no springs could cause this in some cases ?

edit- How about a pad that the metal backing plate is too long and the fiber part of the pad never makes it fully to the rotor surface ? It flexes and springs back.

 

We bled the system again tonight and could not even get a bubble out of it. The shop owner didn't see the need for trying to increase the pedal rod length after I told him about the high pedal that I had after driving for a while. He wants me to drive it for about a hundred miles and see what happens. Noonie, the only adjustment we could have done to the rod would have been to take the little bit of the stop light switch adjustment out or remove and relocate the stop light switch altogether. Shop owner didn't have an answer as to why the pedal was so low at start up but he was hoping it would improve with driving it. He thought we would get some more air out of the lines tonight but that didn't happen. The one thing that has improved the brakes to this point is installing the new master cylinder that he furnished. I'll drive it and see what happens. I had hoped that we would have found the magic bullet that would have instantly fixed everything but I guess we'll wait a while longer. At least I'm riding !!!

 

You can see in the earlier pic that the stoplight switch is adjusted all the way up, so if that's the same as yours, it's factory and should be OK.

Here is some info on bedding the pads, it always helps a bit. I compare it to having grease and oil on the pads and rotors, you would be pushing harder and have a little lower pedal to get enough clamping force to stop.



The last car I did, I had some extra time and wanted to experiment a bit.
I've got a hydraboost setup, but didn't want to use it, it shouldn't be necessary for this weight of car, so I tried this.
The car had no brake problems, new ss calipers, new rubber hoses, old pads, new master. For reference, the braking started less than 1/2" from the top and full hard pedal lockup was a little over 1/2 way to the floor, same as oem new.

I switched to a truck master that had 1-1/4" bore ($50.00), rebled, and that brought the full hard lockup to a little less than 1/2 pedal, but also made for a little harder pedal feel and gave a lot better feedback. One could do all the math for the extra master volume and calculate the difference in stroke and pedal force.
The reason why I tested it was because I wanted a less mushy pedal than they originally came with.
Most people couldn't tell the difference looking at the masters.
There are also larger alum masters that fit for around 100 bucks new.
I would do that mod to any I will ever own.

Only difference externally is that that the larger reservoir is in the rear instead of front.

 

Do you think the spring makeup on the truck piston might be different ?

How hard would it be to bore a vette cylinder to 1 1/4 ?

Hmmmm

 

I just installed the front brakes to the larger reservoir as proper and they worked very well.
Master is for 4 wheel disks, so there should be no problems with springs.
Not enough meat to bore the 1-1/8" vette master anyway.

 

Got it.

 

Noonie, the brake switch has about the same amount of adjustment as yours so the rod length should be good.The pads should be bedded in by now as I have made more than 30 stops, some at less than 30 mph and some in the 40 - 50 mph range. The clamping force is really good, better than its been since I've owned the car. My only issue now is the extremly low pedal when I inatally start driving the car until I reach a little speed and apply the brakes a couple of times, then the pedal comes up to an acceptable height.

 

If you still have a low pedal when you start driving and it still gets better after driving then you still have a problem, you don't want the brakes to fail on a panic stop.
The brakes should work 100% from the moment you start the car. Keep digging until you find your "magic bullet".

 

I read this entire thread, every post last night. Bunch of great info for bleeding brakes.
But I have to say I feel for Billy, that sucks......damn...
Sounds like it still may point to a booster issue.
My new build does not give me much vacuum at idle, like around 11'' even lower once its in gear. I do not think that is adequate to operate the booster effectively. I am unable to lock up the tires now, was able to with old tired engine with higher vacuum.

I have heard from another source and also read in another post that if you add a NON power manuel master to a booster, this increases pressure maybe. As mentioned above they said it will stand the car on its front bumper since the bore diameter/piston is smaller. Something I am thinking about trying before I go to a electric vacuum pump or Hydraboost.
Maybe Noonie or Roger can chime in on the Manuel (non power MC) with a booster. Worth a try at this point as well as a new booster to try.
Good luck,

 

I've driven manual brake Corvettes with the wrong master on them,in other words someone installed a power brake master on them by mistake. With 2 feet it is difficult to stop the car,feels terrible.
I haven't driven a power brake car with the smaller diameter master cylinder,that I know of.

 

I've got a rain delay here. I didn't drive the car yesterday at all. Today the rain stoped for a good while and the roads dryed enough to take it out but as soon as I got on the road it started to sprinkle so I turned around. The brakes were the same as last time I drove it. Very low pedal at start and it quickly increased to full pedal and good brakes. I know I still have a problem but I have run out of ideas. Will wait for better weather and try to put some miles on it to see what happens. If anyone has any more thoughts on my new problem please chime in.