Poor brakes at low RPM
 

I am new to this forum. I just purchased a 71 Chevelle, I am very disappointed with the braking. I've noticed the power brakes are very bad when at idle and low speeds. This is my 2nd 71 Chevelle and never had issue's with power brakes, although this one is not as stock as my previous. It's got a 400 bored out to a 406 with a pretty beefy cam. Any suggestions on what to look for first? Any possibility it's the booster?

Any suggestions would be greatly appreciated.

john

It's not pulling enough vacuum because of the camshaft. Install a vacuum reservoir canister, that will fix your braking issue relatively cheap & easy.

Welcome to the Chevelle forums John.

A couple questions I have for you would be, do you know how much vacuum you have at idle? Did you check your brake hose to the manifold for leaks? Maybe the check valve at the booster is bad or leaking.

We like to see some pics too.

Mike

There is a canister currently installed. I also notice power steering is effected as well when I first start the car. I will check vacuum at idle as that seems most logical at this point. The carb is a "demon" which I am not all that familiar with since i've only had Holley carbs. Can carbs also effect vacuum?

I appreciate your responses! I've got to get this fixed. Makes me nervous driving it!

Sorry Mike, I went to attach a picture but not sure how that's done? never mind- found how to attach

Welcome and I bet the others are right about too little Vacuum as my old chevy truck was just as you described .

Here is a picture.

Welcome John. Looks like a nice car. I had problems with vacuum. Installed a canister like previously suggested and it cured my problem. My problem had more to do with the bigger cam and carb and when I braked I lost a lot of vacuum. Good luck, and just so you know. If you put your pics into an album on your profile page, you will be able to use the BB code and post larger pictures into the threads.

Grumpy, testing the vacuum what is the proper range at idle that brakes require?

WOW ,that yellow really pops ,I like it .You know we want more pics though ,nice car.

11-12 is usually considered minimum. I think I am making 7-9HG and my power brakes worked but were not the easiest as the pedal was kind of hard at times.

Thanks Flash. It's what sold me. I'll update my profile and get some pics posted. Thanks again!

This. I tend to not like a cam with less than 15-16 inch/hg for a driver/cruiser. (I'm getting old)

I have not yet had the chance to test vacuum yet but I did have a question. While working on the rear end this weekend I noticed the fuel lines coming out of the gas tank, the rubber sections look like they need replacement. I believe one of those lines is a vacuum line is that correct?

I think one line is just a return line ,no vacuum I think .

I'll have to look into it. It seems as though there were at least 3 lines coming out of the tank

Finally got a chance to measure the vacuum. I have about 6-7 HG at idle. I'll need to look into an electric vacuum pump or the hydropump. I am sure there is much opionions about what is best and what people like. the Hydropump seems to solve the problem and not look bad either. Any feedback would be appreciated.

When you say hydro pump, do you mean a hydro boost set up? Like what Turbo Buick Regals had? And what the late model Ford Mustangs employ? It relies on the PS pump and fluid, no vacuum involved. 6-7 is horrible, must be a really lumpy cam with a lot of overlap.

I purchased the car with the rebuilt engine but I did get the cam specs and talk to Comp Cam. intake lift is .508 and exhaust is .511- And yes I did mean hydro boost. Reading other fourms/threads, I hear of it often being used successively albeit has cost associated with it

Lift wouldn't be the deciding factor. What are the specs for ICL, LSA, and the duration at .004" & .050"? Those are the numbers that are used to determine the manners of a camshaft.

I have ICL at 106, LSA at 110, Duration at .050 is 240 and 246. I don't see a spec for duration at .004.

More specs:

duration at .006 283/300

Gross valve lift is .508/.511 but with 1.6 rockers that .542/.545

Comp cam claims I shouldn't have issues with vacuum but I told them at 10.5:1 compression. The seller told me that's the compression but I am trying to research the piston installed and head CC to confirm that. I have not run a compression test yet but I will be doing that next.

Hrmm... maybe you have a vacuum leak somewhere. Still, that's a big cam for a cruiser. JMHO

My 2000 Z28 that ran high 10's - full weight, drag radials, through mufflers... did so with a 228/236 @ .050" duration cam in a 347" LS1. It had great manners and pulled 17-18 inch/hg of vacuum. (factory cam was 20-21 inch/hg for comparison) It was a Crane cam, I had a Comp Cam in it previously that made less vacuum (12-13 inch/hg), made less power, and had lots of cam surge unless I kept the RPM up. That cam was 238/242 @ .050"

I could go on about cam stuff, but I feel we need to figure out your vacuum problems for now.

My main concern right now is getting the brakes improved. If indeed the vacuum has always been low (when previous owner first built the motor-which I am trying to find out) then I can focus on getting a vacuum pump or hydroboost. However, if indeed there is a leak or some other mechanical issue, I’d rather resolve that issue and not put an expensive bandaid on the problem. Other than a leak at the carb or manifold (I check hoses and no leaks), what are some other possibilities?

What is the best way to identify leaks at the carb ? and intake? I don’t want to start ripping things off.