Drum to Disk disappointment - ClassicOldsmobile.com


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Old July 14th, 2017, 04:59 AM   #1
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Drum to Disk disappointment

Short while ago I got this kit assembled to my -69 Cutlass:

https://www.summitracing.com/int/par...123-1/reviews/


and I have to say I'm a bit disappointed. The stopping power was better with original power drum setup. At the same time I have swapped the engine with cam, having 12hg vacuum at idle. Less than with stock engine, did not measure vacuum with that. Also, this kit has smaller booster(9" compared to 11") than original was. What can I do to improve the situation? Go back with original booster, will it fit? Smaller sized master cylinder, I think this one is 1,1/8". Electric vacuum pump? What do you recommend. I would have assumed this kit to be at least equal to drums, but it's not.
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Old July 14th, 2017, 06:41 AM   #2
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Is your proportioning valve adjusted correctly so you have all 4 brakes operating? Is your brake pedal rod in the lower hole and adjusted for full travel? While most power brake systems are designed to run on 13in of vacuum, the brakes should still be ok at 12. Do you have good pedal or is it spongy and still needing to be bled some more? Did you bench bleed the master cylinder?
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Old July 14th, 2017, 02:54 PM   #3
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At the same time I have swapped the engine with cam, having 12hg vacuum at idle. Less than with stock engine, did not measure vacuum with that. Also, this kit has smaller booster(9" compared to 11") than original was.
So you have a double whammy.

Vacuum booster power depends on two things (besides pedal pressure), the area of the diaphragm and the vacuum pressure. Area goes with radius squared. The 11" booster has nearly 50% MORE area than the 9" and thus nearly 50% more pressure. Dropping the idle vacuum from about 17" to 12" ALSO drops pressure by about another 40%, so you have maybe 40% of the original booster force.

The other issue is hydraulic system master and slave cylinder areas. the smaller the M/C and the larger the wheel cylinders, the lower the pedal effort (or the higher the braking force for a give pedal pressure). What are the caliper piston diameters on those aftermarket calipers? Stock were 2.75". Smaller will reduce braking force.
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Old July 14th, 2017, 03:37 PM   #4
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For some reason I thought it was a double diaphram booster. Unfortunately it is not.
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Old July 15th, 2017, 10:32 AM   #5
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Is your proportioning valve adjusted correctly so you have all 4 brakes operating? Is your brake pedal rod in the lower hole and adjusted for full travel? While most power brake systems are designed to run on 13in of vacuum, the brakes should still be ok at 12. Do you have good pedal or is it spongy and still needing to be bled some more? Did you bench bleed the master cylinder?


Adjusting the proportion valve seems not make dramatical change how the brakes feel. Pedal rod is in the lower hole. Changing it to upper hole might help some, by increasing the leverage. Pedal is not soft or spongy. I did not bench bleed the M/C. I'm bleeding with vacuum pump through bleeding screws. I really don't understand the meaning of bench bleed, you have to bleed it anyhow when you have everything assembled.
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Old July 15th, 2017, 10:46 AM   #6
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So you have a double whammy.

Vacuum booster power depends on two things (besides pedal pressure), the area of the diaphragm and the vacuum pressure. Area goes with radius squared. The 11" booster has nearly 50% MORE area than the 9" and thus nearly 50% more pressure. Dropping the idle vacuum from about 17" to 12" ALSO drops pressure by about another 40%, so you have maybe 40% of the original booster force.

The other issue is hydraulic system master and slave cylinder areas. the smaller the M/C and the larger the wheel cylinders, the lower the pedal effort (or the higher the braking force for a give pedal pressure). What are the caliper piston diameters on those aftermarket calipers? Stock were 2.75". Smaller will reduce braking force.

All your points are facts. It does not feel like air in the system, pedal is hard, just the pedal-force needed to stop the car is too much. I have to investigatete possibility to use original drum booster with this kit.. And maybe smaller diameter M/C, 1" is what I'm thinking of. The wheel calipers have 71mm (2,75") pistons. I do not understand why almost every conversion kit has smaller than stock booster.
I have never unassembled M/C. Are the pistons for front and rear circuits same or different in size?
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Old July 15th, 2017, 10:55 AM   #7
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Adjusting the proportion valve seems not make dramatical change how the brakes feel. Pedal rod is in the lower hole. Changing it to upper hole might help some, by increasing the leverage. Pedal is not soft or spongy. I did not bench bleed the M/C. I'm bleeding with vacuum pump through bleeding screws. I really don't understand the meaning of bench bleed, you have to bleed it anyhow when you have everything assembled.
Besides that the upper hole is for manual brakes only, you can't put the rod in the upper hole as the angle is too great and it will break the booster seal. Bench bleeding the master cylinder means bleeding it before installing. The master cylinder is mounted at an angle and may trap air.
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Old July 15th, 2017, 10:55 AM   #8
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I have to investigatete possibility to use original drum booster with this kit..
I've done plenty of swaps and just reused the original booster. The disc M/C bolts right up.

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And maybe smaller diameter M/C, 1" is what I'm thinking of.
That will help.

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I do not understand why almost every conversion kit has smaller than stock booster.
I don't understand why people buy kits with smaller than stock boosters. This is high school physics.

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I have never unassembled M/C. Are the pistons for front and rear circuits same or different in size?
Yes, for these M/Cs. Very few use different size pistons due to the added cost of manufacturing with a step in the bore. The Astro vans were one such application.

And the reason why you want to bench bleed is that when installed in the car, the M/C can be at an angle that traps air in the end of the bore. On the bench is is presumably flat. Of course, jacking the car so the M/C is flat does the same thing.
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Old July 15th, 2017, 10:59 AM   #9
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Besides that the upper hole is for manual brakes only, you can't put the rod in the upper hole as the angle is too great and it will break the booster seal.
Unfortunately, I've seen gorillas do exactly that - and worse!
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Old July 15th, 2017, 11:57 AM   #10
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Adjusting the proportion valve seems not make dramatical change how the brakes feel.
Is the proportioning valve plumbed correctly. They will not work if the pressure is plumbed into the wrong side. Also the reason these kits come with the smaller diaphragm, is they are cheaper and sometimes there is no room for the larger booster. Is the "bullet" inserted in the master cylinder if it needs it?

Last edited by edzolz; July 15th, 2017 at 11:59 AM.
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Old July 16th, 2017, 06:08 PM   #11
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You can purchase or rent a Brake Pressure psi tool, this tool can be used to symmetrically balance Left to Right Disc, Left to Right Drum or all Four Disc or all four Drum. You may have to check with the recommended PSI requirements per the Kit you are using. This tool can find mechanical or human error with the system and eliminates the guess work out of the equation.
Hope this helps.
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Old July 18th, 2017, 01:46 AM   #12
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Thanks for the comments! Because I'm pretty sure there is nothing assembled wrong, I'll start to change components one by one to see the effect. The first one will be smaller diameter M/C. Going from 1.125" to 1". Starting with M/C first, because I'd like to keep the new 9" booster.


If there is something to learn here, DON'T belive in kits sold. I was trying to let myself easy and purhase a kit with everything needed, assuming the part combination is already figured out to work well together. I was wrong. Should have figure out what is really needed and buy everything separately.
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Old July 19th, 2017, 05:22 AM   #13
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And the reason why you want to bench bleed is that when installed in the car, the M/C can be at an angle that traps air in the end of the bore. On the bench is is presumably flat. Of course, jacking the car so the M/C is flat does the same thing.

I' about to change the M/C to a smaller one, and still wondering the bench bleed procedure. Ok, the M/C when assembled is at angle, and you should bench bleed it level. But how the heck you prevent air going in the M/C after bench bleed and during assembly?
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Old July 19th, 2017, 06:57 AM   #14
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But how the heck you prevent air going in the M/C after bench bleed and during assembly?
Physics. With the M/C filled and the the cap and rubber seal in place, there is no vent to allow air into the reservoir, so there is no way for fluid to leak out. Same principle as when you put your finger over the end of a straw and pull it out of the glass full of soda - the soda stays in the straw.

Will some fluid dribble out? Yeah, but very little. Certainly plugs in the outlet ports will help.
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Old July 19th, 2017, 12:25 PM   #15
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If you could get two 2000 psi gauges your gonna ultimately want about 1500-1700 psi in front and 800-1000 in rear on a hard stop. Start there. The rear pressure won't be linier, front should pretty much be. You could do it with the car running and have the gauges screw into the bleeders and have somebody operate the brakes.
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Old July 20th, 2017, 06:23 AM   #16
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Physics. With the M/C filled and the the cap and rubber seal in place, there is no vent to allow air into the reservoir, so there is no way for fluid to leak out. Same principle as when you put your finger over the end of a straw and pull it out of the glass full of soda - the soda stays in the straw.

Will some fluid dribble out? Yeah, but very little. Certainly plugs in the outlet ports will help.


Now that makes sense. Why is that not told in the bench bleed instructions?!? They tell nothing about capping the reservoir when the bench bleed is done.... Should have figured out myself... feel stupid


Joe, do you happen to know was there a manual brake disk/drum cutlass ever made by factory?
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Old July 20th, 2017, 06:43 AM   #17
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Joe, do you happen to know was there a manual brake disk/drum cutlass ever made by factory?
My 70 W-30, for one. This was offered on the 70-72 W-30 and 70 W-31 as part of the W-package. The manual disks were actually introduced in mid-year 1969 on the 69 W-cars.
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Old July 20th, 2017, 11:14 PM   #18
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My 70 W-30, for one. This was offered on the 70-72 W-30 and 70 W-31 as part of the W-package. The manual disks were actually introduced in mid-year 1969 on the 69 W-cars.


I quess factory made them with manual brakes due to low vacuum of the hi-perf engine? Do you happen to know the M/C and front caliper piston sizes? How is the pedal feel and pedal force needed?
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Old Yesterday, 03:22 AM   #19
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I quess factory made them with manual brakes due to low vacuum of the hi-perf engine? Do you happen to know the M/C and front caliper piston sizes? How is the pedal feel and pedal force needed?
Yes, exactly due to the 328/328 cam in the manual trans cars. M/C is 1", pistons in the calipers are the same 2.75" as all others. Pedal ratio is obviously different from power brakes. I never had a problem stopping nor did I ever feel the need for power brakes. I am quite happy with those brakes.
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Old Yesterday, 05:05 AM   #20
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Joe,


The 1970 manual disc brake cars used a 1 1/8 master cylinder bore. The books indicate that they used a 1 inch bore, but that is an error on the part of Oldsmobile.
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