Sensitive power brakes after booster & pedal swap
07-22-2014, 02:38 PM
#1
Race Director
Thread Starter
Sensitive power brakes after booster & pedal swap
2 years ago, I replaced my power brake booster while doing a 5-speed conversion. The brakes have been pretty sensitive ever since.
Basically, the first bit of travel allows for light braking, but there's a transition point where the brakes apply HARD. Basically, it's very difficult to apply moderate braking.
What I know about the car:
* Previous booster (original delco morraine) was OK to modulate.
* New booster is MBM PB9522.
* Master cylinder is 1 1/8" bore
* Fluid in good shape, recently bled.
* Car originally had automatic, now manual. The clevis for the booster is attached to the lower rod on the pedal arm, with the brake switch arm on the upper rod (verified last night).
Anyone dealt with this before?
Basically, the first bit of travel allows for light braking, but there's a transition point where the brakes apply HARD. Basically, it's very difficult to apply moderate braking.
What I know about the car:
* Previous booster (original delco morraine) was OK to modulate.
* New booster is MBM PB9522.
* Master cylinder is 1 1/8" bore
* Fluid in good shape, recently bled.
* Car originally had automatic, now manual. The clevis for the booster is attached to the lower rod on the pedal arm, with the brake switch arm on the upper rod (verified last night).
Anyone dealt with this before?
07-22-2014, 03:06 PM
#2
Melting Slicks
I had a similar issue but it resolved itself..... I went from manual to power and thought maybe the MC bore was causing it.
After new shoes, drums and flex lines, no more problem.
Since you have discs you should have less problems.
It could be pushrod length into booster or MC..... It could be brake pedal clevis adjustment....it could be....???
Id be curious if someone has the def answer.
After new shoes, drums and flex lines, no more problem.
Since you have discs you should have less problems.
It could be pushrod length into booster or MC..... It could be brake pedal clevis adjustment....it could be....???
Id be curious if someone has the def answer.
07-22-2014, 06:15 PM
#3
Race Director
Thread Starter
The clevis attachment on the late C3s doesn't have much adjustment... and the pushrod is just a solid piece of metal, near as I could tell. At least the old one was. I haven't looked at the new one since I installed it.
Wonder how many people have used the MBM booster, and whether or not they used the short pushrod (~2" IIRC) that went with it.
Wonder how many people have used the MBM booster, and whether or not they used the short pushrod (~2" IIRC) that went with it.
07-22-2014, 06:48 PM
#4
Melting Slicks
I wonder if the pushrod you are using is too long. If you have an alternate shorter pushrod, its worth a shot. The more I think about it, It sounds like your pushrod is too long giving you very little travel. Also may mean that the MC plunger is always partially depressed...
07-22-2014, 07:19 PM
#6
Race Director
I will write this: Which DOES and DOES NOT apply to your scenario.
I installed a NEW power booster and master cylinder in a 1972 a long time ago that came with manual brakes.
Pressure bled the new system numerous times and COULD NOT get brakes to save my backside when the engine was running. I pulled this booster/master cylinder OUT and put it in my 1977...where I took the master cylinder/booster out ( which was working PERFECTLY and put it in the 1972. when I bled the 1977 system...no brakes.
Put the new system in my 1977 and had bitchin' brakes.
Well...it had to modify and weld a nut to the clevis and get it right in regards to the amount of nut I DID NOT grind off to get the brakes in the 1972 to work. When I got it right...the 1972 had AWESOME brakes.
It could be something is off a little bit...because if I remember correctly...I only kept about 1/8" of nut to the clevis to get it working very well.
DUB
I installed a NEW power booster and master cylinder in a 1972 a long time ago that came with manual brakes.
Pressure bled the new system numerous times and COULD NOT get brakes to save my backside when the engine was running. I pulled this booster/master cylinder OUT and put it in my 1977...where I took the master cylinder/booster out ( which was working PERFECTLY and put it in the 1972. when I bled the 1977 system...no brakes.
Put the new system in my 1977 and had bitchin' brakes.
Well...it had to modify and weld a nut to the clevis and get it right in regards to the amount of nut I DID NOT grind off to get the brakes in the 1972 to work. When I got it right...the 1972 had AWESOME brakes.
It could be something is off a little bit...because if I remember correctly...I only kept about 1/8" of nut to the clevis to get it working very well.
DUB
07-22-2014, 09:33 PM
#7
Safety Car
I haven't done a vette brake pedal but on most cars, there's 2 holes on the brake lever. The top hole with more leverage is for manual and the bottom was for power brakes that require less leverage on the rod. I mixed it up on an impala and I drove the way you described. If you sneeze or shift your weight while driving it would lock the front wheels.
07-22-2014, 10:07 PM
#8
Race Director
Thread Starter
I haven't done a vette brake pedal but on most cars, there's 2 holes on the brake lever. The top hole with more leverage is for manual and the bottom was for power brakes that require less leverage on the rod. I mixed it up on an impala and I drove the way you described. If you sneeze or shift your weight while driving it would lock the front wheels.
Pictures from my install, used the upper hole for brake pedal switch.
You can see the original automatic arm did this as well:
Last edited by Shark Racer; 07-22-2014 at 10:18 PM.
07-23-2014, 02:43 PM
#11
Race Director
Thread Starter
I'm talking to MBM via email and they recommend shimming the master cylinder out. If that works, then I'll turn the pushrod down a bit. I'm going to give this a try and report back.
07-23-2014, 02:53 PM
#12
Team Owner
What is the status (age and condition) of the flex lines from steel brake lines to calipers? When these get old, they begin to deteriorate INSIDE and can actually restrict the flow of fluid (TO the calipers & FROM the calipers).
Not saying that is a HIGH probability...just that it's possible that it is maintaining caliper pressure longer than you intend. And, if it is doing that, the next 'step' is that it keeps the brake pads engaged with the rotor and generates heat. LOTS of heat!!
Not saying that is a HIGH probability...just that it's possible that it is maintaining caliper pressure longer than you intend. And, if it is doing that, the next 'step' is that it keeps the brake pads engaged with the rotor and generates heat. LOTS of heat!!
07-23-2014, 02:56 PM
#14
Melting Slicks
I once installed a manual master by accident on my power system and had super sensitive brakes, but it sounds like you've confirmed your MC bore is correct.
I'm trying to remember, I may have had a bad booster once that acted as you described. It's hard to remember though as I've been through several boosters on this car in 15 years.
Was there a problem with the booster you replaced? Maybe try going back to the old one.
I'm trying to remember, I may have had a bad booster once that acted as you described. It's hard to remember though as I've been through several boosters on this car in 15 years.
Was there a problem with the booster you replaced? Maybe try going back to the old one.
07-23-2014, 04:43 PM
#15
Race Director
Thread Starter
What is the status (age and condition) of the flex lines from steel brake lines to calipers? When these get old, they begin to deteriorate INSIDE and can actually restrict the flow of fluid (TO the calipers & FROM the calipers).
Not saying that is a HIGH probability...just that it's possible that it is maintaining caliper pressure longer than you intend. And, if it is doing that, the next 'step' is that it keeps the brake pads engaged with the rotor and generates heat. LOTS of heat!!
Not saying that is a HIGH probability...just that it's possible that it is maintaining caliper pressure longer than you intend. And, if it is doing that, the next 'step' is that it keeps the brake pads engaged with the rotor and generates heat. LOTS of heat!!
There's only a 0.007" difference between the two (with the original being longer).
I once installed a manual master by accident on my power system and had super sensitive brakes, but it sounds like you've confirmed your MC bore is correct.
I'm trying to remember, I may have had a bad booster once that acted as you described. It's hard to remember though as I've been through several boosters on this car in 15 years.
Was there a problem with the booster you replaced? Maybe try going back to the old one.
I'm trying to remember, I may have had a bad booster once that acted as you described. It's hard to remember though as I've been through several boosters on this car in 15 years.
Was there a problem with the booster you replaced? Maybe try going back to the old one.
I've confirmed the MC in so much as looked at the outside of it and it says "1 1/8" on it, haven't pulled it and mic'd the inside. Of course, the MC ran fine with the old booster too...
What has caused you to go through so many boosters?
07-23-2014, 04:57 PM
#16
Melting Slicks
Eliminate one factor...put the old booster back on. I seriously doubt its the pedal.
07-23-2014, 04:58 PM
#17
Melting Slicks
Honestly, I can't remember all of the cases, it's been a long time since I've even given it a thought. I know I had one real failure, perhaps one or two that I was chasing better pedal feel and they went back to the store. My current booster has been working well for some time now, it's a later unit with a short pushrod and shallow MC. Sorry I can't be more help.
07-23-2014, 05:20 PM
#18
Race Director
Thread Starter
I tend to agree though, the pedal ratio looks nearly identical. The only way I could absolutely guarantee is to measure it, which I may do if I end up pulling the booster.Honestly, I can't remember all of the cases, it's been a long time since I've even given it a thought. I know I had one real failure, perhaps one or two that I was chasing better pedal feel and they went back to the store. My current booster has been working well for some time now, it's a later unit with a short pushrod and shallow MC. Sorry I can't be more help.
07-23-2014, 06:07 PM
#19
Race Director
Have you shimmed out the master cylinder???
DUB
DUB
07-23-2014, 06:54 PM
#20
Race Director
Thread Starter
