jimbo486

what's the cause for a somewhat mushy brake pedal? when pumped it stiffens and when first applied, it makes the "woosh" sound. is it a bad booster? maybe the system needs to be bled to remove any air? i'm sure my rears are out of adjustment. i wish these "self-adjusters" would self-adjust like their name implies.

John Faughn

It could the vacuum pump , the booster runs off vacuum supplied by the vacuum pump , put gauge on the hose to booster to check .
Normally you should not here the booster , so that may have a leak in system .
Mushy peddle , is usually an issue with the hydrolics .

jimbo486

i checked my vacuum back in september and had a healthy 18in hg. that was the entire system, vacuum pump and booster included. i've heard that the sound i hear when the pedal is applied could mean it's the booster. how can the booster be isolated to check it? i'm not very familiar with vacuum system and how to troubleshoot. if it's a leak, could be something as simple as the hose from the pump to the booster needing replacement?

John Faughn

Its been a while , but some had a vent on the inside of the firewall , and usually that would be a leak in the vent valve .
As for testing , block off what ever component you want to check , put a vacuum to it , it should hold a vacuum , if not , use a rubber hose to your ear & the other end , move it around the suspect part , to listen for leak .

jimbo486

this sounds really stupid, but how can you apply vacuum to it if you've blocked it off or removed it from the system? are you talking about using a house hold vaccum with one of the hose/wand attachments?

John Faughn

Yes with anything that makes a vacuum , need a gauge in also , so you can see what & how long it will hold a vacuum , should be for a min. of 15 min.

jimbo486

ok. so i would imagine anything short of 15 mins. would indicate a leak? it held 18in Hg when i checked the system. but i'm guessing the pump naturally creating vacuum was more than likely the reading i got?

14racing

I would guess with a wooshing sound the booster is bad allowing a vacuum leak when you depress the pedal.

jimbo486

any other thoughts?

Twisted Inc.

to apply vaccum you should use one of those power brake bleeder kits.....the have a hand vaccum pump and gauge right on them.....

RamW350 dually

Speaking about one of those kits, I have one and did do a vacuum test on my truck as I am having just about the exact same, if not close to the same problem that jimbo is having. I have a low, mushy pedal and it almost goes to the floor when I press it, but it does stop the truck fine. I replaced the master cylinder twice within the last 6 months. The first one I replaced mainly due to the fact I still had problems with the brakes and the top cover would always leak. I replaced that one probably 2-3 months ago, bled it before install, and haven't had any leaks from the top cover. No brake lines bad and I installed new rotors and pads on the front axle. I replaced the booster at the same time I did the first master cylinder. But I did a vacuum test about a month ago and going off a reading from the one port on the check valve for the booster I got about 15-20 in Hg. So I can confirm I have a fully functioning vacuum pump and I don't have any leaks in any vacuum hoses/lines. I really think I also have a bad brake booster and last two weekends I went out driving the truck I noticed if I'd floor it from say 5-15 mph to about 40-45 mph and then press the brake pedal it was very firm for that spit second I built engine RPM up. Then when I cruise from about 10 mph to a stop I'm back to this same low, mushy pedal. I've bled the brakes twice, so I can't see it being on the hydraulic side as far as a problem (but still possible). Is there a better place to test for a vacuum reading that can really let me know I have a bad booster or not? It certainly has felt like I have had no "power brakes" for some time and just relying on the mechanicals with no vacuum assist. Thanks.

Sal

jimbo486

thanks for contributing sal. it does sound like yours and my symptoms are similar. to me it sounds like your problem is more your vacuum pump itself. even though you're seeing 15-20in Hg, the feeling of no vacuum assist or "power brakes" it seems to me that it would be the pump. i know there have been plenty of discussions regarding brakes, but hopefully we can get some more input in this thread aimed more towards the problems we're experiencing.

i haven't replaced my master or my booster yet and am hoping that it'll be something as simple as maybe air in the system and would just require a good bleeding. i'd of course start by just bleeding the system and see what that takes care of if anything at all. seems that in most cases i read about a master or booster change doesn't always work as most would assume. when i ran a vacuum test on mine, i had the gauge hooked up in the line coming from the lower diaphragm (which doesn't exist anymore) just before it tee's into the other diaphragm / main line to the booster. i had a good 18-20in Hg. i remember dave (wannadiesel) telling me a way to test the booster, and hopefully he'll correct me if i'm wrong but, i think he said with the engine running to depress and hold the pedal. if it's good, the vacuum should fall but recover after a couple seconds.

i know that the rear brakes don't really actuate enough to assist in braking until a considerable amount of weight is distributed towards the rear or a considerable payload is present. but, could it be possible that my rear shoes are far enough out of adjustment that it would make for a mushy feeling pedal? i'd rather start with the simple and cheap fixes so i think i will start with a simple system bleed and an adjustment of the rear shoes and see how it feels from there.

92roughstock

Start simple and work your way up.
Adjust the rears. I tightened mine up til the wheel wouldnt move, then backed the adjuster off about 5 screwdriver strokes, which equals about 12-15 clicks. "Self adjusting" in this case means crawl under there yourSELF and ADJUST the brakes.

Bleed your brakes, but this time, put a clear hose on the bleeder, and put the other end in a container of old brake fluid (an inch or so will work, just make sure the open end is submerged). This will allow you to see if any air is coming through when you bleed. If there is, you have air getting in somewhere. Start at the AntiStop Valve, then LR, RR, LF RF. On that thought, the RWAL valve can be stuck partially open, causing a low pedal as well.

If the pedal is soft, I'd venture to say the booster is ok. A little hiss isnt uncommon- my last 3 have done it. I think it may be a linkage issue going through the firewall. Mine sounds like a vacuum leak, but could also be rubber squeaking on steel, too.

15inHg sounds a little low. The last truck I tested had 23 or 25inHg, and it seems a few others have been over 20.