1963 buick lesabre all drum brake system 2004 LM7 LS 5.3 out of a Tahoe. Aftermarket universal brake 10” dual diaphragm brake booster. A Chrome Flat Top Dual MC. Routed all new brake lines to the mc. Got the pedal ratio/ geometry the same. Bled the MC as well as the entire system ……but still soft but only when the car is turned on. As soon as the car turns on the pedal is DEAD. it’ll return slowly . My best bet is the booster is faulty and not doing it job ….. i have strong 18-19 HG of vacuum. the car ran great until the booster went out . (i don’t have the Original booster )
Title = soft = mushy soft on (off?) = soft car on pedal is dead = ? = hard, firm, no pedal, normal? return slowly = another different symptom. Disconnect booster. retry. Compare OE booster specs with universal 10 inch dual diaphragm booster specs.
with the car off i have a nice firm pedal but as soon as the car turn on soft pedal like the slightest touch sinks it to the floor.
Gotcha! I thought that was what you were trying to say. I get the strong suspicion the new booster is overpowering the rest of the system. As said, try to find specs on the original, even if you don't have it. I don't know of a way to modify or adjust a booster's assist. You sure don't want to restrict the vacuum or create a leak. So you will probably end up swapping to another replacement or finding a rebuilt stocker.
i’m looking into a replacement but not sure with which one to go with. Dual Or Single ? 9” 10” or 11” was told the bigger the better but i’m not sure
i suspected that the booster on now isn’t working properly my assumption lol. i have a 5.3 LS now as well but maybe ima just go back to a 9” single diaphragm
Explain how the booster is “overpowering” the system? If it’s too much booster the pedal will be extremely touchy but wouldn’t go to the floor.
If the booster is giving too much assistance the pedal would be overly sensitive causing harsh application of the brakes. It would not cause a soft pedal. My guess...??? There is still air in the system. Rebleed at all four corners.
bled the dual mc twice and all 4 corners twice …. not to mention vacuum pump i used as well and still nothing im sure all the air is out
All drum brakes, make sure the adjustment is good. I'll make them a few clicks tight, then bleed, readjust. The bad booster acid test, turn the car off. Pump out all the residual vacuum until the pedal is hard. With a foot on the pedal, start the car. The pedal should sink, but not go to the floor. You may have a bad master. If this was my car, I'd source a factory master booster from a 1967 car, the same model as what you have. Rock Auto has matched masters/boosters. Sometimes they have pictures of the booster mounting to help match it up to your old one.
in my case it does sink to the floor…… and fast as hell!so source a booster /mc combo from a 1967 vehicle? they all came with stock drums?
i have a 63 by the way not a 67. my original system had a single pot master cylinder . i now want to run a dual for that extra safety
What these guys said makes sense, more power won't overcome hydraulic lock. The reason 67 was mentioned is dual master cylinders were federally required. https://us.haynes.com/blogs/tips-tutorials/understanding-your-cars-master-cylinder
since i didn’t get my mc and booster as a KIT i sourced each one separately in hopes they would work . I’m going to purchase a dual mc and 9” single diaphragm brake booster as that was the same size as my original one . when sourcing one …since this car is a B body , all the kits i find for sale are for “A” “F” “X” body’s …. does that matter in my case ? since i’m going to be retrofitting ?
Look up what the original parts were. Find out the specs needed to compare. Check interchange of them. See if they are close to the others. It's research, I don't have that all in my head and the chances of someone else on here knowing the exact solution to this is unlikely if it's been on here this long.
Hey, we would all love for someone to say, "This part, number XXX available from XXX and it's the best and cheapest fix, watch out for this one small detail". The power of the HAMB is often amazing. But some stuff you just have to dig to find.
yes sir .i didn’t know my facts lol. will be sourcing a 9” single diaphragm brake booster and dual master cylinder kit . Instead of getting them separate.
aside from the cheap 50$ brake booster i purchased and several tests later im almost 90% sure its due to mc/booster not matching . other 10% is maybe wheel cylinders , and rear shoes need adjustment so will check that tomorrow
Ok, as long as you’re sure, you should look at how a booster works and see why a booster will/cannot cause a soft pedal first. But you asked for help all while being convinced you already know the answer.
understood . ….. i’m aware a booster cannot cause a soft pedal . but a bad mc can or if it’s not properly matched with the boosters push rod length and that’s what i believe happened in my case . comparing this aftermarket mc to my original one (sourced online pics ) .
When you check the rear shoe adjustment make sure the parking brake cable is slackened off and that the parking brake levers on the shoes are not opening the shoes up. Once the rear shoes are properly adjusted then adjust the parking brake. I don't think this is your problem but incorrect adjustment procedures on the rear brakes can cause excessive brake pedal travel.
Nope it still won’t cause a soft pedal and it’s easy to measure. Explain what’s happening in a bad master cylinder to cause a soft pedal please? Or how the incorrect pushrod length will?
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