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power brakes not working

 
41chevy41
V-6 Camaros rule!
Posts: 119
Joined: 10/08
Posted: 02/03/11 06:07 PM

I've done everything I can think of to make my power brakes work. Front disk and drum on the rear. I've replaced the booster and master cylinder { 7" because its mounted on the frame and thats all the room I have }. Bench bled the master cylinder and bled brakes in correct order. I have residual valves 2 in the front 10 in the back. Not that it matters but I have a proportioning valve in the rear line, for what ever reason it doesn't do what its suppose to do. My back has always locked up and slid the tires. If you put it on the lift and apply the brakes you can't turn the front so they are working but on the road the back locks up. Thats why I put the proportioning valve on, it doesn't make any difference the max 57% or anywhere else. I run a vac pump that runs 22" because my cam doesn't make much vacuum. I've traced the lines for kinks and everything looks ok. All new hoses just to make sure they're not collapsing. I've always had a hard pedal since I built it, its like I'm not getting any assist. I've done the brake pedal test and it never moves when I start the engine, like the booster isn't holding and releasing when it starts. It will hold vacuum for days. I put the old master and booster on another car and it works fine. I've had it to the shop and they just shake their heads any advice would be really appreciated.  
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chevelle83
I mow my lawn and find Chevys
Posts: 2280
Joined: 11/09
Posted: 02/03/11 07:36 PM

have you tried reversing the lines on the master cylinder???,,,, I wander if they could be wrong.  although not likely but sounds like you covered everything else.  

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41chevy41
V-6 Camaros rule!
Posts: 119
Joined: 10/08
Posted: 02/04/11 04:56 AM

I'm willing to try anything, I'll which them this week end and get back to you. Thanks  
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41chevy41
V-6 Camaros rule!
Posts: 119
Joined: 10/08
Posted: 02/05/11 06:13 PM

chevelle83 I changed the lines and it didn't make a difference. Isn't the only difference that the disk brake side is larger to compensate for pad wear? I'm thinking its the geometry of the pivot points. I can't understand why the pedal doesn't go down once I've pumped the brakes and then turn on the vac pump.  
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chevelle83
I mow my lawn and find Chevys
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Joined: 11/09
Posted: 02/07/11 12:10 PM

another suggestion,,,, try removing the brake lines from the master cyl and with the vacuum pump off, then pump the brakes noting the amount of effort it takes to press the pedal.


then simply turn on the pump and press the pedal if the pump is working right and the diaphram in the booster is good the pedal should go with little more than the push of a finger.  

 did the instructions give a pedal ratio??  

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chevelle83
I mow my lawn and find Chevys
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Joined: 11/09
Posted: 02/07/11 12:12 PM

another thing,,,,, are you sure that the pedal is not slightly preloading the master cylinder pistion??

is there slight amount of pedal play between the rod and pistion?  

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41chevy41
V-6 Camaros rule!
Posts: 119
Joined: 10/08
Posted: 02/07/11 07:19 PM

Hi-chevelle83, The pedal isn't preloading the MC I checked that. I have made some headway today.'' I think''. Does this make sense, I backed the adjustment to the drum brakes off a little. A buddy said he thought that they were locking up because I had them adjusted up so close to the drum that they were stopping me before the front could do anything and believe or not the pedal felt better. I also lenghten the brake lever arm the booster rod bolts to. Before the booster rod had to have a slight angle up to mate up to the arm. I read where the ratio for power brakes should be 4:1. Mine was 5:1, by adding 1" to the arm and moving the hole 1/2" down from the other hole I now have a 4:1 ratio. Now it has a straight line in and out of the booster and I actually have a litte down movement on the pedal when the pump comes on. I can't get it out of the garage right now to test it do to the weather, maybe in a day or two I'll get back to ya. Thanks for your help  
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waynep7122
I mow my lawn and find Chevys
Posts: 1130
Joined: 08/09
Posted: 02/07/11 07:59 PM

how about the caliper bore...
how about the wheel cylinder bore...

if the rears are locking up before the fronts....   you need to reduce the braking force applied... not loosen the adjustment...

what a LOT of people who are not auto parts guys don't know...   that there are A LOT of wheel cylinders that will interchange... but have different bore sizes...   the manufacturers changed the brake bias on the same cars with different options but the same size brakes by changing the wheel cylinder bore sizes...

this is a LOT harder with disc brakes on the rears as you don't have any options for piston bore size...

reduce the rear wheel cylinder bore size....  

one last thing...   and i know you know the difference..    there are 2 different lengths of brake linings on many model brake shoes....  these are the self energizing type with a floating adjuster at the bottom..

the short lining goes toward the front of the car... the long lining goes to the rear...

get this mixed up and it can cause the wheels to lock up...

why...  the brake cylinder expands both shoes..  the rotation of the brake drum in the forward direction causes the primary shoe to grab the drum surface.. this exerts pressure on the secondary shoe... the rotation of the drum forces the secondary shoe into the anchor at the top..  the rotation then increases the force on the primary shoe forcing the secondary shoe harder into the drum.. so it takes less force to stop the car...  the harder you push the harder the self energizing effect happens...

warning...   most brake and parts counter guys will be totally against selling you something that is NOT written down in the catalog..    they are trying to cover their ................


so.. do you know what the application of the rear brakes was... and the bore size...  and what brands are available at your local parts store...  as numbers vary between brands...  

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waynep7122
I mow my lawn and find Chevys
Posts: 1130
Joined: 08/09
Posted: 02/07/11 08:01 PM

one additional thing...

what size tires are you running front and rear.....

i am not talking wheel size.. i am talking actual P???/??/??


have you ever weighed the front end and the rear end ...  by driving the fronts only on a truck scale.. then the rears...  

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redneckjoe69
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Posted: 02/07/11 11:57 PM

maybe you have a bad proportioning valve?    70% front is a good starting point.   maybe your vacuum pump isnt working correctly?   odd problem.  Confused    hope you get it figured out.   take it to a different reputable brake shop & let them scratch they're heads.  

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chevelle83
I mow my lawn and find Chevys
Posts: 2280
Joined: 11/09
Posted: 02/08/11 01:50 PM

41 chevy,,,, I did not think about that, but yes it is possible to put too much preload on the rear drums.  

 I did this years ago and the car did want to lock up the left rear tire too soon, and a re adjustment solved the problem.  however the pedal was unaffected by that.

I always adjust the rear brakes to where the drums will just slide on by hand but no force,,,, if you have to pry the drums off or have to use force to get the drums on then they are too tight.

also you should be able to easily turn the installed drums by hand with just
noticeable resistance.  

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41chevy41
V-6 Camaros rule!
Posts: 119
Joined: 10/08
Posted: 02/08/11 05:46 PM

I had two new proportioning valves {the ones you buy from summit for 40 bucks} on it thinking maybe the first one was bad they are suppose to cut the fluid to the rear brakes by as much as 57%. They didn't make many difference cranked to the 57% position the back still locked up first.I don't know whether they only cut volume or what. I'm reading in a 1975 GM manual about dics and drum brake combos. They used a combination valve, they say it incorporates a rear brake proportioning valve. They also say it has a metering valve section that prevents front brake application until a specified pressure has built up in the hydraulic system, so that the rear drum brake linings contact the drum at the same time the front disc brakes apply braking pressure to the discs. The proportioning section proportions pressure to the rear brakes after a pre-determined line pressure has been reached.  This prevents rear wheel lock up on vehicles with light rear wheel loads or upon high  deceleration stops when a percentage of the weight over the rear wheels is transferred forward.
 I started out with just an MC, booster and brake lines. I've added residual valves and the summit proportioning, do you think its worth installing this combination valve and doing away with the summit proportioning valve just to see what happens? I'm hoping it dries up enough tomorrow to try the geometry change and the backing off of the back brakes I did the other day.  
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skyeking
I mow my lawn and find Chevys
Posts: 2738
Joined: 08/09
Posted: 02/08/11 10:49 PM

Laugh Hi Wayne,
 If you have had any experience with Jaguars then you are spot
 on for the Job of Brakemaster..The possibility of Illegitimate
 interchange on these Tanks is a nightmare..What you see is NOT
 what you will get as far as genuine repairs are concerned..IF
 an Upgrade is possible then 2 at a time on front or back..OBVIOUS.

 Another good idea is to get the original Factory Manual and as far
 as copies and overview types best forget them as they are usually
 worthless when needed most...All the best to another Olde  Blush  Cool
 Codger Confused  Grin  
skyeking

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41chevy41
V-6 Camaros rule!
Posts: 119
Joined: 10/08
Posted: 02/09/11 04:45 PM

The weather was decent enough to try the brakes same old hard pedal and back locks up. New MC and booster, it feels just like the old one so there was probably nothing wrong with it, correct clearance between piston and booster rod, check valve holds, new vac pump comes on I think at 16" shuts off at 22", its 3' from booster with new vacuum hose. New pads on front, calipers move out correctly go back in smooth with C-clamp. Back shoe brakes, short on the front, long on back, adjusted so drum slides on easy. Bled and rebled with vac bleeder.
Pedal does go down when doing booster test but not like my other vehicles do. With them as soon as I start the engine the pedal almost falls on its own. This one you have to watch your foot real close to see if it goes down. It does but the pump is ready to shut off before it moves. My brake pedal to booster ratio is 4:1, the brake pedal does not have pressure on the booster rod till you rest your foot on it. Residual valves installed 2 on front 10 on back, brakes work the same with or without them.
I've tried two proportioning valves which cut the back brakes to 57%, made no difference.
The rear is a complete 73 nova 10 bolt, drum to drum, I don't know the bore size of the wheel cylinders.
Tires are ft. 205/65R15   Rear 255/70R15
 I can't think of anything else to do other than run it off a cliff!!  
41 chevy

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redneckjoe69
I mow my lawn and find Chevys
Posts: 2337
Joined: 03/10
Posted: 02/09/11 05:11 PM

dont jump yet.  Smile     why do you have 10 valves in the back?   & you said the master cylinder & booster work fine on another ride?   no assist?   i understand the low mount.  

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