slade69

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.

oldcutlass

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?

joe_padavano

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.

oldcutlass

For some reason I thought it was a double diaphram booster. Unfortunately it is not.

slade69

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.

slade69

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?

oldcutlass

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.

joe_padavano

I've done plenty of swaps and just reused the original booster. The disc M/C bolts right up.

That will help.

I don't understand why people buy kits with smaller than stock boosters. This is high school physics.

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.

joe_padavano

Unfortunately, I've seen gorillas do exactly that - and worse!

edzolz

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?

seansolds

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.
Sean

slade69

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.

slade69

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?

joe_padavano

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.

1970greensupreme

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.

slade69

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?

joe_padavano

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.

slade69

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?

joe_padavano

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.

joesw31

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.

joe_padavano

That may be true, but it makes no sense. The larger bore on the MC will increase the required pedal pressure (though reducing pedal travel). I'm pretty sure the MC in my 70 has been replaced. I wonder if it's 1". I'll have to investigate.

slade69

This is from 442.com:


1970 - 1972 Master Cylinders


There was some discussion earlier regarding the differences between the 1970-1972 master cylinders. Hopefully the following info will help: 1970 Power disc. All models (1st design. Dual bail) 1.125" bore.
1970 Power disc. All models (2nd design. Single bail) 1.125" bore.
1970 Manual disc (including W-30) dual bail. 1" bore.
1970-1972 Power or manual drums. Single bail. 1" bore.
1971-1972 Power disc. (incl. W-30). 1.125" bore. (Same as '70 2nd type).
1971-1972 Manual disc (W-30). Dual bail. 1" bore.


I believe it has originally been 1". Also found a good story of a guy testing different components trying to achieve good manual brakes:


http://www.hotrodders.com/forum/manu...dy-212363.html


According to that, 1" seems to be the best size with 2,75" calipers.... 7/8" does not have enough volume.

midnightleadfoot

The small booster is so those with tall valve covers and/or have to lash valves..


Get a dual diapham 9" booster and call it a day..
And maybe a vacuum pump..
Many use the s-10 booster.. not sure what years..
Low vac. cam I'd have gone hydroboost..
Is the one way check valve in the vacuum system?
What pads came with this kit ? are they street pads or street track pads that need heat in them to get the stopping out of them..
Did you break in the pads correctly?


Stainless steel brake company has been around a long time..
I'd pause and drop the idea of try'n to re engineer their kit.. and find what is wrong..
not bedding in the pads correctly, and lack of vacuum , I'd be looking at.. along with what type pads came with the kit..
I had a kit that didn't stop well at all, but after some heat in it would put you through the windshield if you were not careful..


good luck..

joe_padavano

^^^this!

slade69

I got the 1" master assembled and 1,125" out. After bleeding the system, I got brakes again. They are not exellent, but good enough for safe driving. Now I just have to use them and get the pads bedded in properly.


In some point I will try manual set up. I have everything similar(dimension-wise) to W-31 manual brakes. Just have to remove the booster , relocate the master and use the upper hole in the pedal.


I really don't want the hassle with hydroboost (expensive) or electric vacuum pump. Sure they are working solutions, but I like to keep things simple as possible.


After all, the original power drums are quite ok, when in good working order. Drum brakes self boosting feature helps a lot, and that is something missing from disk brakes. You need more line pressure with disks, that's for sure.


I think many enthusiast may have too optimistic expectations when converting from drums to disks, at leat I had

slade69

You are probably right. The kit might work ok with high vacuum (17hg or more) stock engine. It's even told in the instructions that this kit needs 17hg of vacuum pressure. Stupid me I was not doing my homework properly. Kit was not right for my application with 12hg at idle. Smaller M/C was the cure for me. Bigger area booster, for example my stock drum booster, or another 11" one would help even more.
The pads in the kit are probably regular street ones. I'm still in the process of breaking them in. I assume the bite will better when this is done.
The loss of kit's original iron 1,125" M/C is not a big thing, replaced it with much nicer and much lighter Wilwood 1". I'm getting something back by selling the ugly one.

slade69

I belive this is the plastic part in the booster where the manifold vacuum hose connects to? Correct me if I'm wrong?